Implement log in rust

[espindola]

With global variables implemented, we can probably implement log in rust itself.

[msullivan]

pcwalton wound up implementing a general purpose log using shapes in C++ instead. Or do you mean a different part of it?

[msullivan]

Well, we could still move the code that checks the flag and figures out whether to log in rust.

[brson]

pcwalton says it would be easy at this point to turn the log statements into library functions

[brson]

but to preserve line numbers we would have to make it a syntax extension

[catamorphism]

I think #3627 subsumes this. Reopen if you disagree.