Adding ccache-like output caching to dmd

[Per Nordlöw]

Has anyone considered integrating into a `dmd` a ccache-like 
caching of output files indexed by digests based on

- environment variables,
- process arguments which, in turn, decide
- input file contents (including import files detected upon first 
uncached compile)
- dmd compiler binary fingerprint
- ...probably something more I missed

Initial call stores that list alongside content hash and 
resulting binary(s).

If not, would anyone have any strong objections against adding 
this?