add capacity to svm_byte_array

[YaronWittenstein]

Motivation

In case the bytes pointer of svm_byte_array points to data of a Vec,
we need in the Rust side to use Vec::from_raw_parts

This is required mainly for managing Receipts.

Since the length of a Vec might be strictly less than its capacity, it's better not to assume anything about the relation of these two fields and add explicit capacity field to the svm_byte_array.

[YaronWittenstein]

Using the new Vec#leak instead of Vec#into_raw_parts makes this issue not necessary.