nimrod-lang: New and improved formulas

This pull request attempts to improve Homebrew's support for the Nimrod language by supplying two new and improved formulas. The explanation for the changes are in the commits themselves, and I'd also be happy to answer any questions about this here.
It is not officially supported or recommended. nimble installs to a user's home directory by default, and as far as I know, the Nimrod compiler can only detect a nimble installation if it is in the user's home directory. So we have no way of supplying a nimble installation through Homebrew unless we use this wrapper to bridge the gap.

If you really don't like the wrapper, we can wait on including nimble in Homebrew until Nimrod's compiler gets support for a relocatable nimble.