Mark emit_to_memory as unsafe, and introduce a safe emit. (#281)

* 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.

lib/simplejit/src/backend.rs

@@ -103,7 +103,7 @@ impl<'simple_jit_backend> Backend for SimpleJITBackend {
         // Ignore traps for now. For now, frontends should just avoid generating code
         // that traps.
         let mut trap_sink = NullTrapSink {};
-        ctx.emit_to_memory(ptr, &mut reloc_sink, &mut trap_sink, &*self.isa);
+        unsafe { ctx.emit_to_memory(&*self.isa, ptr, &mut reloc_sink, &mut trap_sink) };
 
         Ok(Self::CompiledFunction {
             code: ptr,