VHDL-2008 functions, keywords and operators

[levi-cc]

Good to have VHDL-2008 features like :

• functions : to_x01, to_x01z, to_ux01, to_01, to_string, to_hstring, to_ostring, minimum and maximum
• keyword : context
• operators : ?=, ?/=, ?<, ?>, ?<= and ?>=