Add the tagset-symbol-getter operator (GetSymbols), bump required Corpus2 version to 1.0.2 since new features are needed.

It is more intuitive that way, e.g. equal(2,3) is never true, equal(3,3) is always true.
Nowhere is only equal to Nowhere, and Begin or End are equal to nonspecial positions
depending on current position in the sentence context.

Added parentheses for all "general" operator rules (string_operator, bool_operator, symset_operator and position_operator).

'unify the names of the rules').

This has to be done explicitly for Windows and I think doesn't hurt for Linux.
It doesn't find the dependencies on Win, may be because we're still building
STATIC there, tho I'm not sure if DLLs wouldn't have this problem, either.

* reset_values function to set all Values to their defaul-construct state
* read-only access to all variables of some type via get_all<T>
* Variables cloning
* convenience functions: get_or_throw, get_value
Value subtypes now contain a value_type typedef and require a const value_type& get_value() const member function.