Simple bank-aware malloc scheme

[AndraBisca]

This PR introduces a SimpleBankAwareAllocation scheme to AIEAssignBuffers.cpp that does round-robin allocations across available memory banks. This new memory allocation scheme will be enabled by default while the previous scheme, now called BasicAllocationScheme, can be enabled via the --basic-alloc-scheme flag in aiecc and --basic-alloc in the lowering pass using:

aiecc.py --basic-alloc-scheme
and
aie-opt --aie-assign-buffer-addresses="basic-alloc"

This PR also restructures AIEAssignBuffers.cpp such that future allocation schemes may be easier to add.

This is an improved version of #1108.

[denolf]

LGTM