Compiler should have more comments

[jyasskin]

I'm having a lot of trouble navigating the bootstrap compiler because almost none of the functions or types are commented, and there's no glossary of what abbreviations like ext, iso, carg, etc stand for.

[graydon]

I'm writing up a guide-to-the-source presently, will commit later this afternoon. Sorry for the sparsity so far.

[graydon]

Added a little guide as src/boot/README, let me know if there are more parts you want elaborated-on.

[pcwalton]

As stated, this applies to rustboot, which is long since dead and is kinda vague; closing for now.