Use is_val_statically_known to optimize pow

In the dynamic exponent case, it's preferred to not increase code size,
so use solely the loop-based implementation there.
This shows about 4% penalty in the variable exponent benchmarks
on x86_64.

library/core/src/lib.rs

@@ -170,6 +170,7 @@
 #![feature(internal_impls_macro)]
 #![feature(ip)]
 #![feature(is_ascii_octdigit)]
+#![feature(is_val_statically_known)]
 #![feature(isqrt)]
 #![feature(link_cfg)]
 #![feature(offset_of_enum)]