Support the `sass:meta` module

[kaj]

There should be complete support for the sass:meta module. Some are done in #80, some improvements in #110 and #113.

• load-css (partial support done in Basic support for meta.load_css. #131)
• calc-args (done in Impement sass:meta functions calc-args and calc-name #126)
• calc-name (done in Impement sass:meta functions calc-args and calc-name #126)
• call
• content-exists (minor problems with error reporting remains)
• feature-exists (the function is correct, but not all expected features are implemented)
• function-exists (a problem with functions available from multiple global modules remains)
• get-function (a problem with functions available from multiple global modules remains)
• global-variable-exists (a problem with variables available from multiple global modules remains)
• inspect
• keywords
• mixin-exists (a problem with mixins available from multiple global modules remains)
• module-functions
• module-variables
• type-of
• variable-exists (a problem with functions available from multiple global modules remains)

Common problem:

• Handle things that exists globally from multiple modules.