Userspace `@connected` methods

[tjpalmer]

Add some decoration (maybe just @connected?) tells temper to copy in backend code to call instead of translating temper source.

(This is part of the request in #130, but that also suggests a different form of customizing per-backend logic.)

Some TODO items:

• Backend tests to present better and to lock in translation somewhat
• Finalize translation/api decisions
• Frontend errors for unsupported features: rest params, generics, generators, anything but top-level functions (where we might be more flexible in the future, but I've worked within a subset for now just to make the common cases simple)
• Interp/fallback code
• Per-backend dependencies in config.temper.md
• Stable internal/unexported method names
• Verifying things work in standard builds, not just funtests

[tjpalmer]

Related PRs so far include #457, #465, and #470.