allow adjacent prefix/postfix operators of the same precedence

[medovina]

The documentation for mparser's expression parser says

Adjacent prefix and postfix operators of the same precedence are not well-formed. For example, if (-) denotes prefix negation, --x is not a well-formed expression (if (--) does not denote an operator on its own).

This limitation is inconvenient. As a feature request, it would be nice to remove this limitation, so that e.g. - - x would mean - (- x) for a prefix operator -.