Implements simple compiler for defasm and import forms.

Sybilant can now compile files with defasm and import forms.  This is
pretty much a straight one-to-one translation into assembly, but some
analysis occurs:

  - Any symbol use is verified against all defined symbols.

    Top-level forms are processed as if having been entered at a REPL,
    so any reference to a symbol defined by another top-level form must
    come after the defining form.

    Within a defasm form symbols may reference labels defined later in
    the defasm form.

  - Labels defined within a defasm form are local to that form.  Other
    defasm forms cannot reference them.  In the assembly output they are
    prefixed with a '.' to ensure they are local.

There is a sybilant-test directory for compiler test cases, and a script
to run them.  Each *.syb file in sybilant-test is compiled and run.  It
succeeds if its exit code is 0, and fails otherwise.

A defasm.syb test file is included that calculates the 10th Fibonacci
number and exits with an exit code of 0 if that number is equal to 55.

Also included are a clean script and a script to build an
arbitrary *.syb file as an executable.