Consolidate and rename Parser.* built-ins

* btl/builtins.typer (Parser_default, Parser_newest): Remove.
(Reader_parse): Rename from Parser_custom.

* btl/do.typer (get-decl): Add `ctx` arg.  Use Reader_parse.
(do): Pass it Elab_getenv.

* src/eval.ml (parser_default, parser_newest): Remove.
(reader_parse): Rename from parser_custom.