Add resize_unchecked to AccountInfo

[febo]

Problem

The current behaviour of AccountInfo::realloc allowing to skip zero-initializing the memory is deprecated. Programs should not rely that newly-allocated memory is zeroed. A new resize method was introduced to zero-initialize the data, although programs might define alternative initialization that do not require the data to be zeroed.

Solution

This PR modifies the realloc implementation to use resize, thus ignoring the zero_init parameter and always zero-initializing the data. Additionally, it adds a resize_unchecked, which can be used when the program performs the account data borrow check and initialization of the allocated memory.

[febo]

This will be refactored to provide a MaybeUninit<&[u8]> return.