Draft: env: fix functions taking constant parameters of access types

LRM19 6.5.2. Interface object declarations:

For an interface constant declaration (other than a formal parameter of the predefined = or /= operator for an access type) or an interface signal declaration, the subtype indication shall define a subtype that is neither a file type, an access type, nor a protected type. Moreover, the subtype indication shall not denote a composite type with a subelement that is of a file type, an access type, or a protected type.

This is most likely an oversight, and these parameters should be variables instead.

This fixes VHDL-Issues#259.