Basic ternary logic implementation.

 * Trit type added instead of Bool type. Possible values are false, true and none.
 * Obsolete Bool type is replaced everywhere. Several functions (e.g. from fs::FilePath) may now return none.

String routines are moved from core::string to core::unicode.

Reimplementation of the fs::link and fs::symlink for Windows.

Iterators for fs::Walker class.