Specialise error message for confidential access

Accessing a confidential method from outside of an object previously
reported that there was no such method. The exception remains the same,
but the message notes that the method is present but not accessible
instead.
Add up to 20 repeats of mixfix control structures

The definitions of many control structures in the prelude were defined
multiple times for an increasing number of parts, to produce control
structures of larger size. Now these definitions exist internally, and
each control structure may be used with up to twenty uses of the
repeating part. The elseif() part has been renamed to elseIf() for
consistency, and try() catch*() finally?() now has generic parameters
over the result. The MatchFailure object has been moved into the
exception definitions as a result.

The match result objects have had their result method renamed to value,
for consistency with the corresponding paper. A Nothing pattern has been
introduced which matches nothing, to indicate definite failure.