Fix undefined behavior

Transmuting from an immutable to a mutable reference is undefined
behavior in Rust, with the exception of UnsafeCell which tells the
optimizer to not draw too many conclusions. Sadly UnsafeCell::get cannot
yet be used in statics (see https://github.com/rust-lang/rust/issues/51718),
so this works around that by making the statics into functions and
making C macros to call those.