Add mockable home fn and Env trait

[LucioFranco]

This PR follows up and replaces on #23 which intended to add mockable
functions to the home crate. This PR follows in similar vain except
for a few changes.

• All mockable fn and the Env trait moved into a new env module to
  keep the docs clean from all the extra methods that are usually not
  required for most users of the crate.
• Rename the functions to be with_env instead of just having _from.

The goal of this PR is to enable rustup and hopefully cargo in the
future to be able to write tests which mockout the environment while
still going through the core logic of this crate.

cc @rbtcollins @kinnison

[rbtcollins]

I don't understand the changes vs what I did, and I don't have the time right now to build up context again to understand them - sorry.

But, if this permits the existing code in rustup, or equivalent code with different spelling, to work, then great, thank you.

[LucioFranco]

@rbtcollins sorry I could have explained that part better!

Basically, its your code but moved into a module and I renamed a few fn to what I personally found a bit easier to grok. So the actual technical changes are still the same. The reason I moved them into their own module is to make it so the main docs page is a bit lighter on fn in the docs and should make it easier for most users to find what they need but then reach into the env module to use the special fn + the trait you wrote to mock the diff std::env calls.

Hopefully that makes sense! Totally understand on the time part as well :) Thanks for checking in.