vhdl: introduce Arena type parameter
vhdl: introduce Arena type parameter

Add an `Arena` type parameter to `FromAst`, which is used to allocate
nodes. Modify `Slot` to pass this arena around for node construction,
such that e.g. packages have a way to allocate their child declarations.

fabianschuiki/moore,fabianschuiki/moore,fabianschuiki/moore,fabianschuiki/moore,fabianschuiki/moore