Add final attribute

[rossberg]

This adds a spec of the final attribute for experimentation:

• In the abstract syntax, it is possible to write sub $t* ... or sub final $t* ....
• In the binary format, sub final is encoded as -0x32, with the same format as sub.
• When validating a sub definition, none of the supertypes may be final.
• The non-sub shorthands (and therefor existing function types) are reinterpreted to final.

Due to the latter, a subtypable struct or array must now be written explicitly, with an empty list of supertypes:

(type (struct))  ;; final, no supertypes (shorthand)
(type (sub final (struct)))  ;; same in expanded form
(type (sub (struct)))  ;; non-final, no supertypes
(type (sub $t (struct)))  ;; non-final, with supertype
(type (sub final $t (struct)))  ;; final, with supertype

[rossberg]

Added interpreter implementation and test suite changes & extensions.

[rossberg]

Taking the liberty to land this, since I will already be out tomorrow, for 3 weeks.