Remove most unsafe code

[DemiMarie]

Most unsafe code should (eventually) be replace with safe code to provide Rust’s security benefits.

[folkertdev]

We have, though for compatibility with the libbz2 interface, some unsafe is necessary. When compiled with the stdio flag turned off, the amount of unsafe is really very modest (mostly centering around input/output buffers and a custom memory allocator).

Is there anything else you had in mind?