build: mark no_std as legacy feature flag

The `no_std` feature flag has no effect, anymore (as already documented
in `README.md`). Document this in `Cargo.toml`, but retain it for
backwards compatibility.

Note that for better cross-package composability, an `std` flag would
likely be preferred in the future, over a `no_std` flag. The former
allows multiple packages with different preferences on this feature to
be combined in a single build, while the latter does not.

Hence, suggest that `no_std` as feature flag is a legacy compatibility
flag, and will not be used in the future.

Cargo.toml

@@ -24,6 +24,8 @@ compiler_builtins = { version = "0.1", optional = true }
 
 [features]
 default = []
-no_std = []
 bench = []
 rustc-dep-of-std = ['std', 'core', 'compiler_builtins']
+
+# Legacy, now a no-op
+no_std = []