realloc: Copy old_size number of bytes (not new_size) into new allocation

[fitzgen]

When reallocating, 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
   beacuse 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.

Fixes #69