Expand the datatypes in rust port

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Context

The current implementation of rust loader only supports a limited subset of Rust datatypes into MetaCall type conversions.
The goal of this issue is to track which types are already supported and which ones are still missing.
This also helps unify the metacall protocol representation with the Rust side.

Currently working

From testing the example script:

-bool → METACALL_BOOL
-char → METACALL_CHAR
-i16 → METACALL_SHORT
-i32 → METACALL_INT
-i64 → METACALL_LONG
-f32 → METACALL_FLOAT
-f64 → METACALL_DOUBLE
-String → METACALL_STRING
-Vec → METACALL_BUFFER
-Vec → METACALL_ARRAY
-HashMap<String, T> → METACALL_MAP

Missing types

The following types likely need implementation or verification in the (rs_port) and value conversion layer:

Primitive types

• i8 / i128
• u8 / u16 / u32 / u64 / u128
• f64
• bool
• char

Optional and Result types

• Option
• Result<T, E>

Collections

• Vec (other than i8)
• Vec<Vec>
• Tuple types
• Structs

Complex types

• Enums
• Nested maps / arrays
• Null / unit type ()