Implement libsql_bindgen in Rust for interoperability between libSQL and WebAssembly

[psarna]

WebAssembly modules are seldom coded by hand. Instead, Rust, C++, AssemblyScript and any other language compilable to Wasm is used for that purpose. In order to facilitate creating user-defined functions in Rust that work properly with libSQL types (https://www.sqlite.org/datatype3.html), we should create a crate which exposes a libsql_bindgen macro. This macro can take care of translating Rust types directly into something that libSQL understands, which would allow users to write user-defined functions for libSQL in native Rust.

Example (pseudo-Rust, subject to change):

#[libsql_bindgen]
fn append_42(input: String) -> String {
    input + "_42"
}