Support backtracking and Checkpoints across nodes
#68
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Rowan, and hence CSTree, is designed around hand-written parsers. In particular, the APIs for *building* trees require that each token is recorded only once. Some parsers, and especially parser combinators, use backtracking instead, where the same token may be seen multiple times. To support this, add a new `revert` function which discards all tokens seen since the last checkpoint.
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Makes
Checkpoints work across node boundaries and adds a new methodGreenTreeBuilder::revert_toto support backtracking parsers.This PR supersedes #67. Context from that PR: