Refactor and extend coercion and type validation

[dslh]

Addresses #1164, #690, #689, #693.
Depends on solnic/virtus#343

Grape::ParameterTypes is renamed Grape::Validations::Types
to reflect that it should probably be bundled with an eventual
grape-validations gem. It is expanded to include two new
categories of types, 'special' and 'recognized' (see
'lib/grape/validations/types.rb'). CoerceValidator now makes
use of Virtus::Attribute::value_coerced?, simplifying its
internals.

CustomTypeCoercer is introduced, attempting to standardize
support for custom types by decoupling coercion and type-checking
logic from the type class supplied to
Grape::Dsl::Parameters::requires.

JSON, Array[JSON] and Rack::Multipart::UploadedFile (a.k.a
File) are designated 'special' types, for which special
implementations of Virtus::Attribute are provided.

Instances of Virtus::Attribute built with Virtus::Attribute.build
may now also be passed as the type parameter for requires.
A number of pre-rolled attributes are available providing coercion
for Date and DateTime objects from various formats in
lib/grape/validations/types/date.rb and date_time.rb.

[dslh]

I've had a go at making better use of virtus to make type validation a bit more flexible. I would say this PR needs few more things done before it should be accepted:

• The pending PR to virtus needs to be accepted. Pointing virtus at the PR branch in the Gemfile resolves all regression errors. Could probably work around this in the mean time if necessary.
• Is Grape::Validations::Types a good namespace name?
• See CustomTypeCoercer#infer_type_check. There's support there for more involved type checking than just value.class == primitive, but I'm unsure what the conventions should be. This should be agreed on and documented under Grape::Dsl::Parameters#requires. Alternatively, arbitrary type-check control can be achieved by supplying a Virtus::Attribute subtype overriding value_coerce?, see lib/grape/validations/types/json.rb and file.rb. This could also perhaps be better documented.
• The pre-rolled attributes in lib/grape/validations/types/date.rb and date_time.rb are as-yet undocumented. That's because these 'convenience' types must currently be accessed as type: Grape::Validations::Types::Date::Iso8601... I'm unsure of the best way to make these more readily available to ParamsScope.
• I'm not 100% sure that coercion was tested to the point where I could confidently say no regressions have been introduced around validating nulls, blank strings, and the like. Removing the special type check for Rack::Multipart::UploadedFile for example, did not cause any failures until I added to the spec. This might need to be reviewed.
• I do not have a good understanding of how some of the more exotic uses of the type parameter interact with the automatic documentation system. If anyone can point me in the right direction I would be happy to revise the code as needed.

I think what's missing now is a coerce_from: :param or %w{array of params} and you have a type system that can be adapted to most requirements.

[dblock]

Should be just custom? to match the other names maybe. Not feeling strongly. Maybe some other methods should be private and this would not.

[dblock]

I really like this code and I think it's important to get some or all of it in. I would vote to even monkey-patch Virtus in Grape until there's a version out there with the PR, or implementing a work-around in Grape that explicitly does what that PR does. Either way, bring this PR to a green state and I'll merge it trusting our specs not to introduce any regressions.

[dslh]

Tidied up a bit:

• lib/grape/validations/types/virtus_collection_patch.rb duplicates the virtus PR, works with both fixed and unfixed virtus versions so we can remove it after a fixed virtus gem is published.
• Logic from CoerceValidator#converter moved to module function Types.build_coercer. Not crazy about the name, coercer_for maybe?
• I've removed Types::Date and Types::DateTime, the specs seem to run foul of bugs in certain ruby versions and it's not really core grape stuff anyway. If there's any interest I could put together a 'grape-formats' gem with these and a few other more or less common formats.

Most of the inference that CustomTypeCoercer does on the type parameter still isn't documented in the README. I don't have any strong opinion as to how it should work, maybe leaving it buried will encourage proposals.

[dblock]

You've added type: File, right?

[dslh]

Yep, doc'd and spec'd.

[dblock]

Maybe should be added to changelog as a line, like that we support File now out of the box.

[dblock]

I'm merging this, great work. Lets iterate on top of it.