Moving .h files to src/, where they belong.

--HG--
rename : buffer.h => src/buffer.h
rename : helpers.h => src/helpers.h
rename : keywords.h => src/keywords.h
rename : nodes.h => src/nodes.h
rename : tokens.h => src/tokens.h
Moving spec files to their own directory.

--HG--
rename : gcc.excspec => spec/gcc.excspec
rename : pcc.excspec => spec/pcc.excspec
rename : tcc.excspec => spec/tcc.excspec