don't allow setting state variables in a templates _build function

[blmalone]

Discussed here.

good safety feature to track is to assert this. Implementation could be something like:

vm.dumpState() -> filter for storage in address(this)
do the build method
dump state again after build and keccak256(initStateOfThis)) == keccak256(endStateOfThis)

Essentially, if _build tries to set any state variables, they will be reverted before _validate runs. This behavior is unintuitive and potentially unsafe. It occurs because we revert to the previous snapshot after _build completes. See this line.