sdk: Avoid memory initialization on create program address

[febo]

Problem

As described on #79, create_program_address and try_find_program_address are currently initializing the memory for the derived pubkey. This is unnecessary since the memory is initialized by the syscall.

Solution

Avoid memory initialization by using a MaybeUninit array.

Fixes #79

[publicqi]

try_find_program_address too