vim: Add :Run and :Expand commands

Define a command :Run to compile and run the current file. This supports
unnamed buffers (by writing to a temporary file). See the comment above
the command definition for notes on usage.

Define <D-r> and <D-R> mappings for :Run to make it easier to invoke in
MacVim.

Define a command :Expand to display the --pretty expanded output for the
current file. This can be configured to use different pretty types. See
the comment above the command definition for notes on usage.

Create an autoload file and put function definitions there to speed up
load time.
Rename :Run and :Expand to :RustRun and :RustExpand
Write documentation for the Rust vim plugin
Add commands :RustEmitIr and :RustEmitAsm