Flaw in `realloc` allows reading unknown memory

When reallocing, if we allocate new space, we need to copy the old
allocation’s bytes into the new space. There are old_size number of bytes in
the old allocation, but we were accidentally copying new_size number of bytes,
which could lead to copying bytes into the realloc’d space from past the chunk
that we’re bump allocating out of, from unknown memory.

If an attacker can cause reallocs, and can read the realoced data back,
this could allow them to read things from other regions of memory that they
shouldn’t be able to. For example, if some crypto keys happened to live in
memory right after a chunk we were bump allocating out of, this could allow
the attacker to read the crypto keys.

Beyond just fixing the bug and adding a regression test, I’ve also taken two
additional steps:

1. While we were already running the testsuite under valgrind in CI, because
   valgrind exits with the same code that the program did, if there are
   invalid reads/writes that happen not to trigger a segfault, the program can
   still exit OK and we will be none the wiser. I’ve enabled the
   --error-exitcode=1 flag for valgrind in CI so that tests eagerly fail
   in these scenarios.

2. I’ve written a quickcheck test to exercise realloc. Without the bug fix
   in this patch, this quickcheck immediately triggers invalid reads when run
   under valgrind. We didn’t previously have quickchecks that exercised
   realloc because realloc isn’t publicly exposed directly, and instead
   can only be indirectly called. This new quickcheck test exercises realloc
   via bumpalo::collections::Vec::resize and
   bumpalo::collections::Vec::shrink_to_fit calls.