Extend declared/actual type check to composite types

[jozanek]

Drops the isAssignableTo / getResultType allowlist helpers from SType in favour of structural equality on the user's ascription. The SigmaBinder no longer overwrites Val.givenType with the body's partial type, so the typer can tell user-written ascriptions from binder scaffolding (Val.tpe still falls back to body.tpe for any downstream consumer). def f(args): R = body is handled via an isFunDef discriminator that compares against the lambda's range.

PR #1061 caught val x: Boolean = 10 and val price: Long = bigInt, but only for 7 primitive types. Coll[T] / Option[T] inner mismatches, tuple element mismatches, SFunc return mismatches, SigmaProp/Box mismatches, and undeclared type names (issue #404 ex 1: val f: XYZ = …) were silently accepted.

Closes #404 and #915