Fix syntax of explicit-implicit args

* typer-mode.el (typer-smie-grammar): Fix grammar for `:=`.
Add if/then/else to the grammar.  Remove `:-` and `:≡`.
* tests/sexp_test.ml: Add test for `:=` and if/then/else.
* tests/eval_test.ml, tests/macro_test.ml: Use explicit implicit args.
* src/grammar.ml (default_grammar): Update from type-mode.el.
* doc/manual.texi (Core Syntax): Only keep `:=` for explicit args.
* src/builtin.ml (get_attribute_impl, declexpr_impl)
(is_builtin_macro): Silence compiler-warning.
* btl/types.typer (length): Pass explicitly implicit parameter.