Refactor and simplify build for utilities.

For coreutils, there are two build artifacts:

  1. multicall executable (each utility is a separate static library)
  2. individual utilities (still separate library with main wrapper)

To avoid namespace collision, each utility crate is defined as
"uu_{CMD}". The end user only sees the original utility name. This
simplifies build.rs.

Also, the thin wrapper for the main() function is no longer contained in
the crate. It has been separated into a dedicated file. This was
necessary to work around Cargo's need for the crate name attribute to
match the name in the respective Cargo.toml.

Cargo.toml

@@ -177,5 +177,5 @@ rand="*"
 tempdir="*"
 
 [[bin]]
-name="uutils"
-path="src/uutils/uutils.rs"
+name = "uutils"
+path = "src/uutils/uutils.rs"