'zig run file.zig' builds a file and stores the artifacts in the global
cache. On successful compilation the binary is executed.

'zig run file.zig -- a b c' does the same, but passes the arguments a,
b and c as runtime arguments to the program. Everything after an '--' are
treated as runtime arguments.

On a posix system, a shebang can be used to run a zig file directly. An
example shebang would be '#!/usr/bin/zig run'. You may not be able pass
extra compile arguments currently as part of the shebang. Linux for example
treats all arguments after the first as a single argument which will result
in an 'invalid command'.

Currently there is no customisability for the cache path as a compile
argument. For a posix system you can use `TMPDIR=. zig run file.zig` to
override, in this case using the current directory for the run cache.

The input file is always recompiled, even if it has changed. This is
intended to be cached but further discussion/thought needs to go into
this.

Closes #466.