Provide an alloc_extend method

[ennis]

It would be great if Bump could provide an alloc_extend method to allocate values from an iterator and return a slice (similarly to TypedArena::alloc_extend). Something in the lines of:

pub fn alloc_extend<T, I>(&self, iterable: I) -> &mut [T] where
    T: BumpAllocSafe,
    I: IntoIterator<Item = T>, 

[fitzgen]

We have alloc_slice_fill_iter and various other alloc_slice_* methods now, so I think we can close this. If not, please open a new issue with specifics!