Add KnownLayout::validate_cast_and_convert_metadata (#349)

This method is a core primitive that other parts of the codebase can
build on to validate casts and compute the appropriate pointer metadata
which should result from those casts. Future commits will build on top
of this one to make use of it.

Co-authored-by: Jack Wrenn <jswrenn@amazon.com>

Cargo.toml

@@ -55,6 +55,8 @@ optional = true
 zerocopy-derive = { version = "=0.7.3", path = "zerocopy-derive" }
 
 [dev-dependencies]
+assert_matches = "1.5"
+itertools = "0.11"
 rand = { version = "0.8.5", features = ["small_rng"] }
 rustversion = "1.0"
 static_assertions = "1.1"