perf: elide bounds checks in release mode

[hawkw]

When indices are returned by the Core::idx_gen function, we can assume
that they are in bounds, since any bits that would make the index out of
bounds for the core's array size are masked out. Therefore, we can
safely use slice::get_unchecked to elide bounds checks.

In debug mode, we assert that the index is in bounds, just in case
there's a bug somewhere.

In conjunction with #50, this means MPSCs should never panic in release
mode.

Signed-off-by: Eliza Weisman [email protected]