* Mark emit_to_memory as unsafe, and provide a safe compile_and_emit.
Mark `Context::emit_to_memory` and `MemoryCodeSink::new` as unsafe, as
`MemoryCodeSink` does not perform bounds checking when writing to
memory.
Add a `Context::compile_and_emit` function which provides a convenient
interface for doing `compile` and `emit_to_memory` in one step, and
which can also provide a safe interface, since it allocates memory of
the needed size itself.
* Mention that `MemoryCodeSink` can't guarantee that the pointer is valid.
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