Fix checked_pow() perf: BigInts can never overflow, so don't need checked math.

[NHDaly]

This commit makes checked_pow(x::BigInt, y::Integer) skip directly to ^(x,y). Same for (Integer, BigInt).

This means we're again using the native C implementation for power, instead of interpreting it in julia via repeated multiplications. Thanks to @mcmcgrath13 for discovering the issue.

Before:

julia> @btime ^($(BigInt(2)), 100)
  56.134 ns (3 allocations: 80 bytes)
1267650600228229401496703205376

julia> @btime Base.Checked.checked_pow($(BigInt(2)), 100)
  326.895 ns (24 allocations: 392 bytes)
1267650600228229401496703205376

After:

julia> @btime ^($(BigInt(2)), 100)
  56.430 ns (3 allocations: 80 bytes)
1267650600228229401496703205376

julia> @btime Base.Checked.checked_pow($(BigInt(2)), 100)
  56.557 ns (3 allocations: 80 bytes)
1267650600228229401496703205376

Co-authored-by: @mcmcgrath13

[mcmcgrath13]

thanks! nice test ❤️