boringssl: new formula

Not sure how you’ll feel about this one Jack/Mike/etc, It’s incredibly messy and isn’t production-ready at this point, although I guess that could be expected in a HEAD-only formula. 

It litters CMakeFiles and CMakeLists.txt absolutely everywhere in the installed directories.
Those can be removed with the unix find utility, but I wasn’t sure how much anyone would love something automating that process for them given how badly wrong the -delete and rm -rf process from the command line can go if anything unexpected crops up. It’s a useful library, eventually. 

Whether it’s at a position where Homebrew is willing to ship it in any shape or form is up to you guys. I’d like to see it in to make playing around with it a little easier, but I’ll understand if there’s a reluctance to touch it.