Safer C APIs

[neysofu]

See #260.

• Use #![deny(unsafe_op_in_unsafe_fn)], which makes auditing unsafe code much easier.
• Use method-based casting on raw pointers rather than as. Behavior is better documented and they protect against NULL deference.
• Try to push unsafe code as high as possible within function bodies to highlight what unsafe operations we do.
• Cast pointers to reference as early as possible as they are the main cause of unsafe.