Add `std::encoding::utf8` module

[mjm918]

Summary

Add UTF-8 encoding/decoding support as part of the std::encoding module.

Parent issue: #48

Functions

Function	Signature	Description
encode	(s: string) -> bytes	Convert string to UTF-8 bytes
decode	(data: bytes) -> string throws DecodeError	Convert UTF-8 bytes to string

Usage

use std::encoding::utf8::*;

var raw: bytes = encode("Hello 世界");
var text: string = decode(raw);

// With error handling
var text: string = decode(invalid_bytes) catch e {
    println(e.message);
    return;
} ?? "";

Implementation Notes

• naml strings are already UTF-8 internally, so encode is essentially a view/copy
• decode validates UTF-8 sequences and throws on invalid bytes
• Use Rust's std::str::from_utf8 for validation

Exception

exception DecodeError {
    position: int,
}

[mjm918]

Implemented in std/naml-std-encoding/src/utf8.rs with encode/decode/is_valid functions and unit tests.