fN::BITS constants for feature float_bits_const
Also enables the feature for compiler_builtins as otherwise this causes a warning and conflicts with the Float extension trait.
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Implementation for rust-lang/rust#151073
Feature flag: `#![feature(float_bits_const)]`
Note that this is likely to conflict with some extension traits, as it has with compiler builtins. However, assuming correct values for the constants, they are either `u32`, the same type, which should not cause a problem (as shown by enabling the feature for compiler_builtins), or a different type (e.g. `usize`), which should cause a compiler error. Either way this should never change behaviour unless the extension trait implemented an incorrect value.
Also note that it doesn't seem to be possible to put multiple unstable attributes on an item, so `f128::BITS` and `f16::BITS` are gated behind the feature flags for those primitives, rather than `#![feature(float_bits_const)]`
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