Improve panic messages for Vec methods

[IgorPerikov]

I've spotted, that remove() panic message could've provided details on what values led to this panic, instead of mentioning condition only. Generally speaking this and some other assert!s in Vec could've been more detailed. If it's okay to step in, I'd like to offer my help.

In case of remove I'd suggest calling this instead

assert!(index < len, "removal index {} should be less than length {}", index, len); 

I find this more informative than assertion failed: index < len

Candidates for improval:

aforementioned remove:

rust/src/liballoc/vec.rs

Line 1034 in 5f13820

assert!(index < len);

insert:

rust/src/liballoc/vec.rs

Line 994 in 5f13820

assert!(index <= len);

drain:

rust/src/liballoc/vec.rs

Lines 1292 to 1293 in 5f13820

	assert!(start <= end);
	assert!(end <= len);

split_off:

rust/src/liballoc/vec.rs

Line 1383 in 5f13820

assert!(at <= self.len(), "`at` out of bounds");

[IgorPerikov]

@Centril Does triaging mean a green light for this suggestion?

[Centril]

@IgorPerikov No, I just apply the relevant labels. :)

[Centril]

@IgorPerikov Though this is a small change with little time required to fix, so I'd just make the PR if I were you. :)

[RalfJung]

Note: #70558 uses the same kind of assertion in swap_remove (as the old check had aliasing issues).

Depending on which PR lands first, we should make sure it gets the same panic message as the others.

[IgorPerikov]

@RalfJung I think it makes sense to follow the message, that is being currently printed, when trying to access element out of bounds via []. Current message template is index out of bounds: the len is {len} but the index is {index}. I followed this idiom to keep it consistent.