This commit reduces space and time overhead for writable memories
to O(write port count) in both cases; implements handling for write
port priorities; and simplifies runtime representation of memories.
After this commit, if NDEBUG is not defined, out-of-bounds accesses
cause assertion failures for reads and writes. If NDEBUG is defined,
out-of-bounds reads return zeroes, and out-of-bounds writes are
ignored.
This commit also adds support for memories that start with a non-zero
index (`Memory::start_offset` in RTLIL).
This results in further massive gains in performance, modest decrease
in compile time, and, for designs without feedback arcs, makes it
possible to run eval() once per clock edge in certain conditions.
whitequark