Switch compiler macros to CONSTANT-FORM-VALUE instead of CL:EVAL

[samuel-hunter]

WIP. Fixes #344.

CONSTANT-FORM-VALUE probably ought to belong in a separate package so that CFFI-SYS can access it without a circular dependency.

[samuel-hunter]

All uses of CL:EVAL are switched out except for the CFFI-SYS macros in cffi-*.lisp. Two ideas I've had for moving CONSTANT-FORM-VALUE to let these macros use it as well are:

• Create a package cffi-sys-util that both cffi-sys and cffi can import from (would exposing a new package be problematic?)
• For each cffi-*.lisp, split them into head and tail files and have CONSTANT-FORM-VALUE loaded in-between (Prevents exposure of a new package, but doubling files here feels like a code smell here)

I'm leaning on option 1. Any thoughts?

[sionescu]

8629618 refactors the package definitions and exports CONSTANT-FORM-P and CONSTANT-FORM-VALUE from CFFI-SYS.

[samuel-hunter]

Switched out functions in cffi-sys. This should be ready for another round of reviews