Add `std::datetime` and `std::metrics` standard library modules

[mjm918]

Description

Add two new standard library modules for date/time operations and performance measurement.

---

std::datetime

Time and date utilities.

Function	Signature	Description
now_ms	() -> int	Current Unix timestamp in milliseconds
now_s	() -> int	Current Unix timestamp in seconds
year	(timestamp_ms: int) -> int	Extract year from timestamp
month	(timestamp_ms: int) -> int	Extract month (1-12)
day	(timestamp_ms: int) -> int	Extract day of month (1-31)
hour	(timestamp_ms: int) -> int	Extract hour (0-23)
minute	(timestamp_ms: int) -> int	Extract minute (0-59)
second	(timestamp_ms: int) -> int	Extract second (0-59)
day_of_week	(timestamp_ms: int) -> int	Day of week (0=Sun, 6=Sat)
format_date	(timestamp_ms: int, fmt: string) -> string	Format timestamp to string

use std::datetime::*;

fn main() {
    var ts: int = now_ms();
    println("Year: {}", year(ts));
    println("Date: {}", format_date(ts, "YYYY-MM-DD"));
}

---

std::metrics

Performance measurement and benchmarking utilities.

Function	Signature	Description
perf_now	() -> int	High-resolution monotonic time in nanoseconds
elapsed_ms	(start_ns: int) -> int	Milliseconds elapsed since start
elapsed_us	(start_ns: int) -> int	Microseconds elapsed since start
elapsed_ns	(start_ns: int) -> int	Nanoseconds elapsed since start

use std::metrics::*;

fn main() {
    var start: int = perf_now();
    // ... work ...
    println("Took {} ms", elapsed_ms(start));
}

Implementation

Follow the existing StdModuleFn pattern used by std::io, std::random, std::threads:

1. Typechecker: Add "datetime" and "metrics" entries in get_std_module_functions()
2. Runtime: Add extern "C" functions in namlc/src/runtime/
3. Codegen: Register symbols and add dispatch in cranelift/mod.rs
4. Docs: Update language-reference.md with new modules