Allow implementing CHECK expressions in Wasm

Described by @xfbs in https://github.com/libsql/libsql/discussions/8#discussioncomment-3798887

The idea is to allow people to code more complex check expressions in any language compilable to WebAssembly and have an interface which makes it easy to register such checks. The code sample proposed in discussion above was:

#[libsql::constraint]
fn check_is_ipv4(data: &[u8]) -> Result<(), ConstraintError> {
    Ipv4Addr::try_from(&data)?;
    Ok(())
}

and it's a great example of why this can be useful - users can rely on standard library implementations of things like IP validation and get a working CHECK expression.

This is an umbrella issue, because it has a few prerequisites, some shared with #1, in arbitrary order:

• Define a Wasm<->libSQL ABI, which specifies how libSQL types should be expressed in WebAssembly functions:
  https://github.com/libsql/libsql/issues/16
• Create helper libraries for handling this type translation in a few popular languages - with Rust leading the way:
  https://github.com/libsql/libsql/issues/15
• Add a user-defined helper function (registered with sqlite3_create_function) which runs a WebAssembly function - e.g. it could take the WebAssembly module body as a blob in the first parameter, then the function name, and then forward the rest of the parameters to the WebAssembly model as is:
  https://github.com/libsql/libsql/issues/17
• Add syntactic sugar for calling a Wasm function without using the runner above - e.g. creating a system table for storing compiled functions indexed by their names, adding a CREATE FUNCTION SQL command for compiling and registering it, and modifying the path for looking up user-defined functions to take a peek into ones created with CREATE FUNCTION:
  https://github.com/libsql/libsql/issues/18

Start by reading the linked discussion and prerequisite issues #16, #15, #17, and #18 to understand the proposed ABI, type-translation libraries, Wasm helper function, and CREATE FUNCTION syntax. The umbrella issue is done only when these prerequisites support implementing CHECK expressions through registered Wasm functions.