There isn't any reason for this feature to be exposed or part of the
build script. Move it to a separate crate.
We will also want more tests that require some support functions; this
will create a place for them.
* Use macros for more division/array checks
This commit moves over more array accesses to the `i!` macro to avoid
bounds checks when debug assertions are disabled. This is surfaced from
rust-lang/compiler-builtins#360 where recent changes in codegen units
has caused some bounds checks to not get elided in release mode. This
also adds a `div!` macro to work around rust-lang/rust#72751.
* Don't test/bench our shim crate
It's not intended to run all our tests
