allocator_api: missing grow_zeroed implementation

[baloo]

I believe the implementation made in #92 could use a specific grow_zeroed implementation

If I read https://github.com/rust-lang/rust/blob/ee88f46bb5e27c4d9f30326e69ff2298dcbeecb1/library/core/src/alloc/mod.rs#L253-L277 correctly, that could use more effective one.
As currently implemented, it grows by new_layout.size() where it could grow by only new_layout.size()-old_layout.size().