feat: Implement parts of the extism runtime in WebAssembly (#384)

This PR adds the `kernel` directory which contains a port of the Extism
memory allocator compiled to WebAssembly and removes
`runtime/src/memory.rs` completely.

Being able to re-use memory functions as a WASM module allows us to
begin to experiment with porting Extism to new runtimes!

This is in a draft state while I'm verifying some of these changes.

Cargo.toml

@@ -5,3 +5,4 @@ members = [
   "rust",
   "libextism",
 ]
+exclude = ["kernel"]