std.hash_map: use 7 bits of metadata instead of 6
std.hash_map: use 7 bits of metadata instead of 6

we only effectively need 1 control bit to represent 2 special states for
the metadata (free and tombstone)
this should reduce the number of actual element equality tests, but since
it's very low already, the impact is negligible

zig-lang/zig,andrewrk/zig,zig-lang/zig,andrewrk/zig,zig-lang/zig