Rollup merge of #44097 - Xaeroxe:clamp, r=burntsushi

Add clamp functions

Implementation of clamp feature:

Tracking issue: https://github.com/rust-lang/rust/issues/44095
RFC: https://github.com/rust-lang/rfcs/pull/1961

src/libstd/lib.rs

@@ -249,6 +249,7 @@
 #![feature(cfg_target_vendor)]
 #![feature(char_error_internals)]
 #![feature(char_internals)]
+#![feature(clamp)]
 #![feature(collections_range)]
 #![feature(compiler_builtins_lib)]
 #![feature(const_fn)]