Implement compile time function evaluation (CTFE) #11621
Description
This is a big gap in Rust's current support of zero cost abstractions, and something that puts us behind languages such as D or C++. This would for example allow us to deprecate the std::uint::{bits, bytes} statics in favor of using the std::mem::size_of function. It would also allow for the use of overloaded operators in constexprs, which would in turn allow us implement the base numeric arithmetic operators within the standard library, as opposed to relying on compiler magic (see #11526).