Use sexps rather than pdecls

* src/lexp.ml (lexp_unparse.sdecls): Skip pexp_u_decls.

* src/elab.ml (instantiate_implicit): Use Eval.varname.
(lexp_decls_macro): Don't return a new context any more.
Return a sexp rather than a list of pdecls.
(lexp_check_decls): Take a list of sexps rather than a list of pdecls.
Simplify the macro call case since it can't return a new ctx any more.
(lexp_p_decls): Take a list of sexps rather than a list of pdecls.
(sform_letin, default_ectx.read_file, _lexp_decl_str):
Pass the sexps directly to lexp_p_decls.

* src/debug_util.ml (format_source): Pass the sexps directly to
lexp_p_decls.
(mdecl, lexp_detect_recursive): Remove rather than fix.
(main): Remove "pexp" debugging because there's no pexp any more.
Remove "merge-debug" debugging because it's broken and it's not clear
how to fix it.
Pass the sexps directly to lexp_p_decls.

* src/debug.ml (debug_pexp_decls): Remove; there's no pexp to debug
any more.
(debug_pexp_print_all): Remove unused.

* src/REPL.ml (ipexp_parse): Use sexp rather then pdecl.
(_raw_eval): Pass the sexps directly to lexp_p_decls.

* src/pexp.ml (pdecl, pexp_p_decls, pexp_u_decl, pexp_u_decls)
(pexp_decls_all, _pexp_decl_str, pexp_decl_str): Remove.