Add Vec::bump method

[mattfbacon]

This PR adds a way to get the Bump allocator backing a Vec, for example to make more allocations if the Vec contains other bump-allocated data structures.

One common example of this is a bump-allocated recursive data structure. Consider:

use bumpalo::Bump;
use bumpalo::collections::Vec;

struct Tree<'bump> {
	value: i32,
	children: Vec<'bump, Self>,
}

impl Tree<'bump> {
	fn new(value: i32, bump: &'bump Bump) -> Self {
		Self {
			value,
			children: Vec::new_in(bump),
		}
	}

	fn add_child(&mut self, value: i32) {
		self.children.push(Self::new(value, self.children.bump()));
	}
}

Without this PR, Recursive would be forced to carry an extra shared reference to the Bump.

[fitzgen]

Thanks!

Would you be interested in adding Box::bump and String::bump as well? Note the the box version should be a static method to match the std Box::allocator method.