Macros

[osa1]

Since we'll have some form of constant expressions that can be evaluated at compile time (#13), it may make sense to have some functions/expressions/special forms that can only be evaluated in compile time and that can return new declarations, statements, and expressions.

These should be flexible to allow at least some of the basic trait deriving, like Debug, Eq, etc.

Related: rust-lang/rfcs#3698