some cleanup after FFI improvements

[kchanqvq]

This keeps the nice idea of JS emulation while cut down some kludges from the implementation. Hopefully uncontroversial :)

• simplify bootstrap process. ffi.lisp is back to target, emulation goes to compat.lisp. read-j helpers in compat.lisp no longer needed, read-j goes to read.lisp.
• cleanup some extraneous conversions, close Why there's both CONVERT-XSTRING and CONVERT-PROPERTY-KEY? #587
  convert-xstring is inlined to jsstring (under no other circumstance it should be used). Fix a dangling convert-xstring which should have been convert-property-key
• we used to have many tests (need to keep sync with literal!) in convert for the branch that outsource to literal, then there's a cond in literal that has no catch-all branch (will return NIL if falls through, too bad!). I changed it so that catch all branch in convert outsource to literal (all atomic value without special meaning should rightfully be handled this way), then catch all branch in literal errors.

[davazp]

Looks great. Feel free to merge. I'll rebase bootstrap tomorrow

[kchanqvq]

Thanks, good night :) and congrats for getting JSCL to build itself!

[davazp]

we used to have many tests (need to keep sync with literal!) in convert for the branch that outsource to literal, then there's a cond in literal that has no catch-all branch (will return NIL if falls through, too bad!). I changed it so that catch all branch in convert outsource to literal (all atomic value without special meaning should rightfully be handled this way), then catch all branch in literal errors.

The default with NIL made me waste a couple of hours of work in the bootstrap. The error is much safer.