lang F1 2.5: contextual typing for multi-param closure literals

An untyped lambda (a, b, c) => ... now takes each param's type
positionally from the expected Closure(T0, T1, T2) -> R signature, for
heterogeneous param types, in both assignment and argument position.

Previously only the first param (or all-same-typed params) resolved:
lowerLambda's signature loop applied contextual typing into params, but
the return-type-inference temp scope and the body param binding both
re-resolved each param via resolveParamType -- which defaults an untyped
(inferred_type) param to s64. So b in Closure(s64, string) bound as s64
and b.len errored. Both sites now read the already-resolved signature
types params.items[user_param_base + i].ty (user_param_base skips the
pre-populated ctx/env slots).

Regression: examples/201-closure-contextual-params.sx.

Note: a generic return $R inferred through a closure-typed parameter is
still unresolved (folds into Phase 4 function monomorphization); concrete
returns work.

tests/expected/201-closure-contextual-params.exit

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

tests/expected/201-closure-contextual-params.txt

@@ -0,0 +1,3 @@
+cb=15
+r=15
+q=6