Also, save dynamic allocations (plus their memory overhead).
This is realized by storing the relocation information, source position
table, and protected instruction information together in one "metadata"
byte array.
For each of the three components, we just store their size, such that
the accessors can return the respecitive {Vector} views as before.

This makes each WasmCode object 24 bytes smaller on 64-bit
architectures. It also saves a few more bytes per code object because
less padding is needed for the individual allocations, and each dynamic
allocation comes with some constant memory overhead.

Since the protected instructions will just be stored in a byte array
now, some APIs are refactored to just return that byte array directly
(instead of an array of {ProtectedInstructionData}). This also
simplifies serialization and deserialization, and will allow for
switching to a more compact representation in the future.

Drive-by: Add some more checks to {Vector::cast} to protect against
  undefined behaviour.

R=ahaas@chromium.org

Bug: v8:10254
Change-Id: I81ca847023841110e3e52cc402fcb0349325d7af
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2078545Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66596}