Fix unsoundness in Hash{Map, Set}'s Collect implementation

[moulins]

These implementations accept arbitrary S: BuildHasher, but don't trace the value at all.

This adds an extra S: 'static bound to prevent smuggling Gc pointers through the hasher. (All 'real' hashers are 'static, so this is better than adding a Collect bound w.r.t. third-party hashers support)

[kyren]

Ahhhh good catch!