* src/elab.ml: Make symbols into shorthands for `typer-identifier SYM`

(newMetalevel, newMetatype): Take a lexp_context i.s.o elab_context.
(infer_varref): New function, extracted from `infer`.
(infer): Make symbols into shorthands for `typer-identifier SYM` and
blocks into shorthands for `typer-block BLOCK`.
(check): Same rewrite for symbols.
(sform_identifier): New functions, using code extracted from `infer`
and `check`.
(register_special_forms): Use it to implement `typer-identifier`.