Build system organization and documentation

[j-hui]

I realize that I didn't leave behind much top-level documentation, and there's some additional cleanup/touch ups I'd like to add to this.

Not ready for merging at time of writing; I'll indicate when it is.

[j-hui]

Wow I really butchered this PR title. Hopefully this new one clarifies things a little more.

This PR now includes commits toward #12, by allowing the top-level Build.mk to be included from outside of this project. I would like to close that issue in this PR, as long as I don't hit any snags (even if that oversteps the original intention of this PR, which was primarily to provide better documentation and organization).

Will update the top comment with a TODO list for myself

[j-hui]

Maybe I'm trying to knock down too many issues with this "documentation" PR, but it looks like I have the beginnings of a solution for #12 in here too (which I will summarily document as that finalizes). Sorry for the lack of focus in this PR.

[j-hui]

With this, one can write an SSM program like this:

-- in MyFn.hs
import SSM
import Cli (compileCli)

myfn :: SSM ()
myfn = ...

-- | boilerplate to compile myfn to file
main :: IO ()
main = compileCli myfn

and "invoke" the compiler as follows:

stack --stack-yaml /path/to/ssm-edsl/stack.yaml runghc MyFn.hs

which will write the compiled C program to myfn.c (using the entry point of program to derive the C filepath). It also supports the argument -o to explicitly specify the filepath:

stack --stack-yaml /path/to/ssm-edsl/stack.yaml runghc MyFn.hs -- -o mycustomname.c

This allows us to easily write a Make rule for it, something like:

%.c: %.hs
    stack --stack-yaml $(SSMDIR)/stack.yaml runghc $< -- -o $@

[Rewbert]

It looks great! Nice documentation for the test & build setup.