Macros and Signatures in mzc

When mzc compiles a Scheme file, macros and signatures are expanded away. Macro and signature definitions in a source file are evaluated (so the macros and signatures can be used later in the source file), but the definitions are not preserved, so loading the compiled file will not redefine the macros or signatures.

Elaboration-time expressions (i.e., begin-elaboration-time and begin-construction-time expressions) are also evaluated by compilation; the S-expression result of an elaboration-time expression is compiled in place of the elaboration-time expression.