This is intended to fix an OSS-fuzz build failure detailed here:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=32817

Fixes #757

By using a boxed slice instead of a vector, we can shrink the size
of the `Inst` structure by 8 bytes going from 40 to 32 bytes on
64-bit platforms.

PR #760

This improves the precision of the "expression too big" regex
compilation error. Previously, it was not considering the heap usage
from Unicode character classes.

It's possible this will make some regexes fail to compile that
previously compiled. However, this is a bug fix. If you do wind up
seeing this though, feel free to file an issue, since it would be good
to get an idea of what kinds of regexes no longer compile but did.

This was found by OSS-fuzz:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=33579