add equivalent of racket's begin-for-syntax

Created by: WreckedAvent

Context:

I want to be able to write private helpers for my macros. Currently, the only way to do this is to have another module which you import. This seems less-than-ideal to me and involves some weirdness like import ... for syntax. I suggest having a syntax block which puts all of the statements into the correct phase to be used in macros.

Example:

syntax {
  let next = ctx => ctx.next().value
  syntax inc = ctx => #`${next(ctx)} + 1`
}

inc 10

would compile to

10 + 1

Considerations:

• @jlongster mentioned a forSyntax accepting an IIFE to achieve this instead. This seems less intuitive to me.
• The import ... for syntax could be implied within a syntax block
• syntax identifier seems kind of awkward to me inside of a syntax block but not sure if there's any better ways to approach it