rs: add DisasmBuffer api to reuse the cs_insn allocation further

[jiegec]

Add DisasmBuffer struct to wrap cs_insn allocation. It calls cs_free when dropped.
Add disasm_iter_with_buffer, create_buffer and into_buffer functions to ease buffer reuse.
The other disasm* functions are not removed yet.

[jiegec]

A possible alternative to this is to add another lifetime and use mutable reference to access DisasmBuffer: &mut DisasmBuffer. So into_buffer is no longer needed (but relying on dropping the iterator upon reuse). But the user must hold DisasmBuffer in a variable then. I am not sure which is better.

[tmfink]

I think we need to take a &mut DisasmBuffer here to ensure there is exclusive access and to allow reuse:

We want to make sure the following is prevented:

let buffer = ...;
let mut insn_iter1 = cs.disasm_iter_with_buffer(code1, addr1, &mut buffer);
// unsound since we are mutating buffer from different iterators
let mut insn_iter2 = cs.disasm_iter_with_buffer(code2, addr1, &mut buffer);
let insn1 = insn_iter1.next().unwrap();
let insn2 = insn_iter2.next().unwrap();
let insn3 = insn_iter1.next().unwrap();

While still allowing:

let buffer = ...;
for code in [code1, code2, code3] {
    let mut insn_iter = cs.disasm_iter_with_buffer(code2, addr1, &mut buffer);
    let insn = insn_iter.next().unwrap();
}

[jiegec]

Updated.