Add integration with zerocopy crate

The zerocopy crate provides traits and functions for safely transmuting types to/from bytes. It is an important building block for high-performance serialization in many designs. It is also a very well-maintained crate with high standards for quality and its maintainers are actively engaged with the Rust Project on advancing the goals of the Safe Transmute working group.

The zerocopy crate defines the FromZeroes trait, which specifies that a type can be safely constructed from a buffer containing an all-zeroes bit pattern.

This PR adds two new methods to Bump: new_zeroed and new_slice_zeroed. new_zeroed allocates space for T (where T: FromZeroes) and returns &mut T. This avoids the "placement new" problem in Rust, which causes problems when attempting to allocate large types in the heap, such as [u8; 0x10000]. For most containers, such as Box, the type is briefly constructed in a temporary on the stack, then a heap allocation is done, then the value is moved into the heap allocation. If T is large enough, it can cause stack overflow.

The Bump::new_zeroed function avoids this problem by simply allocating space for T directly in the heap, then filling it with zeroes, then casting the allocation as &mut T and returning it. The FromZeroes constraint ensures that this is sound.

The new_slice_zeroed function similarly allows allocating slices directly in a Bump.

fitzgen / bumpalo

Add integration with zerocopy crate #253