Cosmetic restructuring of syntax/modcode.

This work is preparation for exporting more functionality from this module.
Renamed variables for clarity and to eliminate shadowing.
Turned lets into defines.
Split a helper function out from get-module-code in syntax/modcode.

The get-module-path function finds the compiled or source file corresponding
to a module; get-module-code reads and compiles the contents of this file.
Exported two more functions from syntax/modcode.

The get-module-path function was described in the previous commit.
The get-metadata-path function constructs the paths used to saved .zo files
and so forth; e.g. PATH/compiled/NAME_rkt.zo.
Add tests for get-module-path.
Added documentation for get-module-path and get-metadata-path.