Re-enable Miri symbolic alignment checking

[joshlf]

In #406, we disabled Miri's "symbolic alignment checking" feature, as it was causing problems (for a description of the issue, see rust-lang/miri#306). Miri added the ability to "promise" alignment in rust-lang/rust#117840, which may allow us to re-enable our use of symbolic alignment checking.

This appears to be the public API we'd use.