More robust handling of prefixes

[aannleax]

Currently prefixes can be used as follows:

@prefix prefix: <https://www.example.com> .

fact(prefix:constant).

This syntax may create ambiguity in certain cases. For example, when allowing '-' within prefixes, it may create confusion with the infix operator for subtraction. I.e. in the program

@prefix my-prefix: <https://www.example.com> .

result(my-prefix:constant) : body(?x) .

my-prefix:constant can be interpreted as the constant https://www.example.com/constant or as the subtraction of the constants my and prefix:constant. Currently, - is disallowed from prefix names for that reason.

Another problem occurs when considering a more natural map syntax that separates keys from values through :. Such cases can create confusion, e.g. consider map { a:b:c } which can either be the mapping of a to b:c or a:b to c.

A possible solution to this would be to include the currently known prefixes in the parser state so that it can be matched before other syntactic constructs are evaluated.