Change IntSet::len() to return u64 instead of usize.

[garretrieger]

A u32 containing intset can have up to u32::MAX + 1 elements in it which can overflow a usize when usize is a u32. Fixes #1057

[waywardmonkeys]

Once this lands, I'll submit a PR that improves the wasm32 CI job. (I was preparing to do that when I found this.)

[behdad]

FWIW, in HarfBuzz we disallow inserting (unsigned) -1, and hence the population always fits in a uint32_t.