Declare and update return.value

[edemaine]

Sequel to #364. I started writing documentation and then added a few intended features:

• let return: T
• return .= init
• return++

Also cleans up the code in a single ReturnValue rule.

[STRd6]

This could pick up a Label on the following line, we may want to disallow __ and enforce same line or use a more specific rule.

[STRd6]

We may also be able to use the return type annotation of a function, if present, to set the default type annotation for the return value.

[edemaine]

We may also be able to use the return type annotation of a function, if present, to set the default type annotation for the return value.

Great idea! I implemented this, which involved improving the AST for types a bit. Not sure I got the best names for things (t is particularly annoying, because type is already taken). Feel free to suggest (or make) improvements.

[edemaine]

4469fab prevents newlines before colons in a lot of cases. (I searched for all __ Colon and changed most of them — the only remaining case is ternary ?:.) This actually fixed a bunch of bugs with label parsing (see new tests in test/label.civet), and didn't seem to break any tests, so it's hopefully OK in practice. If needed, in the future, we could allow newlines before the colon if we then check for a space after the colon to ensure it's not a label.