Rename vec::to_mut/from_mut

[brson]

These are casting functions, not conversion functions. Maybe cast_to_mut, cast_from_mut, or transmut_to_mut, transmute_from_mut (seems to be an established pattern, per core::unsafe).

[nikomatsakis]

There is also a good argument for removing the mutable variants of vectors altogether. There is little need for it, particularly as mutability is inherited through uniqueness. In other words, one can write:

let mut x = ~[1, 2, 3];
x[0] += 1;

and still the type of x is ~[mut int].

[pcwalton]

+1 on removing mutable vectors. A smaller language is a happier language.

[graydon]

Sorta. There's still the question of shared vecs...

[thestinger]

These were removed by 573a31d