To do that, split up the Accounts objects and the instruction
implementations.
GPL code is only used when the "enable-gpl" feature is enabled. That
means compiling the program or running tests need explicit feature
activation now.
This gives us better compatibility with released anchor versions.
Instead of using AccountLoaderDynamic<MangoAccount>, we now use
a standard AccountLoader<MangoAccountFixed>. This will generally work
(except for load_init(), which is dangerous).
A new trait, MangoAccountLoader, provides load_full(), load_full_mut()
etc on the AccountLoader<MangoAccountFixed> to create accessor structs
that can read and write to the dynamic part of the mango account data.