Define READ-FROM-FILE and EXPAND-FILE; simplify makefile; get rid of obj/ directory

[shawwn]

This PR simplifies makefile by getting rid of the obj/ directory. It accomplishes this by adding a new file lumen.l:

(when-compiling
  `(do ,@(read-from-file "runtime.l")
       ,@(read-from-file "macros.l")
       ,@(read-from-file "main.l")))

Additionally, two new global functions are defined: read-from-file and expand-file.

Whenever runtime.l, macros.l, or main.l is modified, bin/lumen.{js,lua} will be correctly rebuilt. (I.e. the new makefile is functionally identical to the old makefile.)