Changed transposed path expressions.

Let p be a path description. The old syntax p^T denoting the transposed
(i.e. reversed) path was replaced by the new form p^-1, a more general
path iteration.

p^-1 is translated as transposed path (only TransposedPathDescription
node is generated)
p^n, where n is a positive integer constant, means n repetitions of p
(only ExponentiatedPathDescription node is generated)
p^-n is translated exactly as (p^-1)^n (ExponentiatedPathDescription
with TransposedPathDescription as parameter are generated)

In GreqlSerializer, TransposedPathExpression nodes are printed as
(p)^-1, iterated path expressions as (p)^n. Hence, parsing and
serializing an input like "p^-3" results in "((p)^-1)^3".