Translate Rust keywords into raw identifiers

[thedataking]

We can be better at picking variable names when the C code uses an identifier that shadows a Rust keyword. The example buffer library uses self as an identifier which is named self_0 in the output. We should emit r#self instead.

[VorpalBlade]

I ran into a variant of this where I had a C file that started with a number. That also needs to be handled similarly.