lang 2.4: protocol-interface method calls on pack elements + conformance fix

Calling a protocol method on a pack element now works: `xs[i].greet()` on a
`..xs: Greeter` pack dispatches to the concrete element's impl, and elements
may be heterogeneous (Dog, Cat). This is the protocol-interface access the
pack is for. (Protocol method decls omit the implicit `self`; impls list it —
the earlier malformed `(self: *Self)` decls were why dispatch looked broken.)

Also fixes packArgConformsTo for non-parameterised protocols: it queried
`protocol_thunk_map`, which is only populated lazily when a protocol VALUE is
built with `xx`, so it false-negatived valid conformers. Now it queries
impl-declaration state directly — `param_impl_map` for parameterised protocols,
or `<ty>.<method>` entries in `fn_ast_map` for non-parameterised ones.

examples/193-protocol-pack-methods.sx (heterogeneous Dog+Cat pack, per-element
greet(), order-independent).

tests/expected/193-protocol-pack-methods.exit

@@ -0,0 +1 @@
+0

tests/expected/193-protocol-pack-methods.txt

@@ -0,0 +1,3 @@
+dog+cat=903
+cat+dog=903
+dog+dog=7