Add a multicall binary

This adds a multicall binary uutils which contains all other utils. Each util
is compiled to a rlib crate and the linked into the main binary. Since generating the
makefile dependencies (via `rustc --crate-file-name`) adds quite a bit of overhead
to each make invocation (~2sec) this is only enabled when the MULTICALL
environment variable is set.

    $ make MULTICALL=1
    ...
    $ build/uutils echo foo
    foo
    $ build/uutils uptime
     14:32:48 up 12 days, 16:39,  7 users, load average: 0.04, 0.24, 0.33
    $ mkdir tmp
    $ cd tmp/
    $ ln -s ../build/uutils uptime
    $ ./uptime 
     14:33:07 up 12 days, 16:40,  7 users, load average: 0.06, 0.20, 0.31