Export Functions Metadata to JSON

Engine::gen_fn_metadata_to_json
Engine::gen_fn_metadata_with_ast_to_json

As part of a reflections API, Engine::gen_fn_metadata_to_json and the corresponding Engine::gen_fn_metadata_with_ast_to_json export the full list of functions metadata in JSON format.

The [`metadata`] feature is required for this API, which also pulls in the
[`serde_json`](https://crates.io/crates/serde_json) crate.

Sources

Functions from the following sources are included:

1. Script-defined functions in an AST (for Engine::gen_fn_metadata_with_ast_to_json)
2. Native Rust functions registered into the global namespace via the Engine::register_XXX API
3. Public (i.e. non-private) functions (native Rust or Rhai scripted) in static modules registered via Engine::register_static_module
4. Native Rust functions in external packages registered via Engine::register_global_module
5. Native Rust functions in built-in packages (optional)

JSON Schema

The JSON schema used to hold functions metadata is very simple, containing a nested structure of modules and a list of functions.

Modules Schema

{
    "modules":
    {
        /* namespace 'sub_module_1' */
        "sub_module_1":
        {
            "modules":
            {
                /* namespace 'sub_module_1::sub_sub_module_A' */
                "sub_sub_module_A":
                {
                    /* functions exported in 'sub_module_1::sub_sub_module_A' */
                    "functions":
                    [
                        { ... function metadata ... },
                        { ... function metadata ... },
                        { ... function metadata ... },
                        { ... function metadata ... },
                        ...
                    ]
                },
                /* namespace 'sub_module_1::sub_sub_module_B' */
                "sub_sub_module_B":
                {
                    ...
                }
            }
        },
        "sub_module_2":
        {
            ...
        },
        ...
    },
    /* functions registered globally or in the 'AST' */
    "functions":
    [
        { ... function metadata ... },
        { ... function metadata ... },
        { ... function metadata ... },
        { ... function metadata ... },
        ...
    ]
}

Function Metadata Schema

{
    "baseHash": 9876543210,  /* partial hash with only number of parameters */
    "fullHash": 1234567890,  /* full hash with actual parameter types */
    "namespace": "internal" | "global",
    "access": "public" | "private",
    "name": "fn_name",
    "type": "native" | "script",
    "numParams": 42,  /* number of parameters */
    "params":  /* omitted if no parameters */
    [
        { "name": "param_1", "type": "type_1" },
        { "name": "param_2" },  /* no type name */
        { "type": "type_3" },   /* no parameter name */
        ...
    ],
    "returnType": "ret_type",  /* omitted if () or unknown */
    "signature": "[private] fn_name(param_1: type_1, param_2, _: type_3) -> ret_type",
    "docComments":  /* omitted if none */
    [
        "/// doc-comment line 1",
        "/// doc-comment line 2",
        "/** doc-comment block */",
        ...
    ]
}