Read implementations should only write into the buffer passed to them,
but have the ability to read from it. Access of uninitialized memory can
easily cause UB, so there's then a question of what a user of a reader
should do to initialize buffers.

Previously, we allowed a Read implementation to promise it wouldn't look
at the contents of the buffer, which allows the user to pass
uninitialized memory to it.

Instead, this PR adds a method to "freeze" undefined bytes into
arbitrary-but-defined bytes. This is currently done via an inline
assembly directive noting the address as an output, so LLVM no longer
knows it's uninitialized. There is a proposed "freeze" operation in LLVM
itself that would do this directly, but it hasn't been fully
implemented.

Some targets don't support inline assembly, so there we instead pass the
pointer to an extern "C" function, which is similarly opaque to LLVM.

The current approach is very low level. If we stabilize, we'll probably
want to add something like `slice.freeze()` to make this easier to use.

Co-Authored-By: sfackler <[email protected]>

Co-Authored-By: sfackler <[email protected]>

Co-Authored-By: sfackler <[email protected]>

Co-Authored-By: sfackler <[email protected]>