Implement libsql_bindgen in Rust for interoperability between libSQL and WebAssembly

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"
}

Start with the proposed libsql_bindgen crate and the pseudo-Rust append_42 example in the issue; the payload names no existing files or tests. Define what Rust types must map to libSQL types and what the macro's supported interface should be, then verify that the example can be used for a WebAssembly user-defined function.