RFC: macros for disabling bounds checks

Provides `@inbounds expr` and `@boundscheck true|false expr`. The former is just a shorter form of the latter.

The gains here are not huge; the best case is generic matmul, which is 2x faster. The `vectoriz` benchmark in perf2 is 10% faster. But, there are more places in Base that could benefit from `inbounds`.