Update to the latest rustc_apfloat

The latest version has a convenient `.unwrap()`. Increase the version so
we can use this.

Author: tgross35

library/compiler-builtins/testcrate/Cargo.toml

@@ -15,7 +15,7 @@ doctest = false
 # `xoshiro128**` is used for its quality, size, and speed at generating `u32` shift amounts.
 rand_xoshiro = "0.6"
 # To compare float builtins against
-rustc_apfloat = "0.2.0"
+rustc_apfloat = "0.2.1"
 
 [dependencies.compiler_builtins]
 path = ".."