usrlocalpkg: Add alias and more wrapper for rust

Add package alias "rust" so that we can easily run tools from the
current rust package which don't deserve a wrapper on their own.

For example

    prun rust miri

Add wrappers for `rustdoc` and `rustfmt`. These are available via cargo,
too, ( `cargo doc`, `cargo fmt` ) but, for example, `cargo help fmt`
refers to "Options passed to rustfmt" so it would be useful, if
`rustfmt --help` would work. Also, external documentation might suggest
users to run these commands directly.

usrlocalpkg/admin/config

@@ -1515,10 +1515,12 @@ CellProfiler-2.2.0-0
 CellProfiler-3.1.5-0
   cellprofiler
 
-rustc-1.52.1-0
+rust=rustc-1.52.1-0
   rustc
   cargo
   cbindgens
+  rustdoc
+  rustfmt
 
 apache-maven-3.6.0-0
   mvn mvnDebug