Fixed memory leak on boxed bytes

[gfusee]

Abstract

The BoxedBytes currently allocates manually a u8 slice before wrapping it into a Box. This has the drawback of making Rust unable to free the memory once the BoxedBytes is dropped.

Changes

• The BoxedBytes::allocate function has been removed
• BoxedBytes::zeros now only uses safe Rust and uses the Vec logic to allocate a slice
• All the usages of BoxedBytes::allocate have been replaced by BoxedBytes::zeros. Since BoxedBytes is not used on WASM contracts anymore this should have no impact on the code. On the RustVM there is no limit on allocations so this has a negligible performance cost