panic safety fix: guard against double drop

[JOE1994]

Hello 🦀 ,
we (Rust group @sslab-gatech) found a memory-safety/soundness issue in this crate while scanning Rust code on crates.io for potential vulnerabilities. This PR contains a fix for the issue.

Issue Description

glsl-layout/src/array.rs

Lines 177 to 199 in 119f5ec

	fn map_array(mut values: [T; $size], mut f: F) -> Self {
	use std::{
	mem::forget,
	ptr::{read, write},
	};
	unsafe {
	// All elements of `result` is written.
	// Each element of `values` read once and then forgotten.
	// Hence safe in case `f` never panics.
	// TODO: Make it panic-safe.
	let mut result: ::std::mem::MaybeUninit<[U; $size]> =
	::std::mem::MaybeUninit::zeroed();
	for i in 0..$size {
	write(
	result.as_mut_ptr().cast::<U>().add(i),
	f(read(&mut values[i])),
	);
	}
	forget(values);
	result.assume_init()
	}
	}

As the comments in the code indicates, the code above needs a patch for panic safety.
If a panic happens within f,
the item copied by ptr::read() can be dropped twice since the ownership of the item is duplicated.

Suggested Fix

I used ManuallyDrop<_> to contain values, in order to guard against double drop.

Thank you for reviewing this PR 👍

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This change is 

[JOE1994]

p.s. the same issue exists for the code that matches the latest crates.io release (v0.3.2)

[zakarumych]

Thanks. This looks exactly how it should be done.
It is possible to add a guard to drop values not yet read. But leaks are safe so it's not mandatory.