Whereas backward-slice matching provides support to limit traversal by
specifying the desired depth level, this pull request introduces support
for limiting traversal with a nested matcher (adding forward-slice
also). It also adds support for variadic operators, including `anyOf`
and `allOf`. Rather than simply stopping traversal when an operation
named foo is encountered, one can now define a matcher that specifies
different exit conditions. Variadic support implementation within
mlir-query is very similar to clang-query.
