Preamble macros introduce code at the beginning of a function body.
Allow them to introduce declarations as well, so long as the macro
declaration properly declares the names it introduces. This allows all
sorts of exciting macros that introduce (e.g.) new local variables
that one can use.
Implement type checking support for preamble macros, which expands the
preamble macros and introduces them at the beginning of the function
body prior to type checking. Ensure that the resulting function bodies
type-check properly, including when composing multiple preamble macros
and with a preamble macro applied to the body of a function that
itself came from a body macro.
Function body macros allow one to introduce a function body for a
particular function, either providing a body for a function that
doesn't have one, or wholesale replacing the body of a function that
was written with a new one.
