Modify the way source files are listed in *SOURCE*

Modify the way source files are listed in *SOURCE*
Allow subdirectories by having a list rather than a keyword in the entry
for a file. For example, the entry:
("foo"
  ("bar" :HOST)
  ("baz" :BOTH))

includes the files `foo/bar.lisp' and `foo/baz.lisp'.

Subdirectories are processed recursively, so arbitrarily deeply nested
directories should work.

A simple DOLIST over *SOURCE* doesn't work any more, as the file list
has to be processed by GET-FILES first. The macro DO-SOURCE should be
used instead. All previous occurances of DOLIST were changed in the
previous commit with the introduction of DO-SOURCE.

Also removed trailing whitespace from compiler.lisp and codegen.lisp to
make my git pre-commit hook happy