Add the 'quasiquote' operator (`qquote`)

Removed `temp_ctx` as it is not used anymore

Changes to be committed:
* src/REPL.ml: temp_ctx
- qquote handling:
	* src/builtin.ml
	* src/eval.ml
	* src/lparse.ml
* src/sexp.ml: new function (sexp_to_st)
* src/prelexer.ml: new function (pretoken_to_str)
* src/lexp.ml: print (Imm(sexp)) correctly