Better optimizer with verilog codegen

[WorldofKerry]

Current problems

• Backwards replace does not respect the nonblocking nature of assignments. E.g., mem_0 may be used even though the flip flop still contains its old value.
• Remove unused variables pass can accidentally optimize away memory stores, as it perceives those assignments as never being used. The current workaround is to have an un-removeable memory node type. However it's hard to say if this should be associated with the node (rename to load/store?) or associated with the variable (e.g. new pointer type), or even both.

Solution ideas

• Make a new nonblocking assignment type that doesn't actually define variables but only declares variables.