Optimize char_try_from_u32 The optimization was proposed by ```````@falk-hueffner``````` in https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Micro-optimizing.20char.3A.3Afrom_u32/near/272146171, and I simplified it a bit and added an explanation of why the optimization is correct. The generated code is 2 instructions shorter and uses 2 registers instead of 4 on x86.