caching AST parsing

any chance of caching the parsing of the AST? i'm particularly interested in this for a watching build process as recast becomes unbearably slow with larger files.

could be included (difficult imo with configurable esprima and options) or an external function. would also be cool if we could cache it to disk (serialize), but that's probably excessive

No file or test is named. Start by tracing recast's AST parsing path and how configurable esprima options enter it, then determine whether a cache can preserve correctness across changing source and parser options. Done should include a defined cache API and tests showing faster repeated parsing without stale or incompatible ASTs.

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