Auto merge of #43055 - est31:stabilize_float_bits_conv, r=sfackler
Stabilize float_bits_conv for Rust 1.21
Stabilizes the `float_bits_conv` lib feature for the 1.20 release of Rust. I've initially implemented the feature in #39271 and later made PR #43025 to output quiet NaNs even on platforms with different encodings, which seems to have been the only unresolved issue of the API.
Due to PR #43025 being only applied to master this stabilisation can't happen for Rust 1.19 through the usual "stabilisation on beta" system that is being done for library APIs.
r? @BurntSushi
closes #40470.