Move AST into on-demand local cache

[jonahbeckford]

Parsed AST in distributable valuestores add a direct entry point into the build system's memory layout.

It is signed by a private key so it is not easy to gain access (and that private key gives access to everything).

Regardless, the reference implementation should keep the AST in a local, non-distributable cache where the parsed AST can be generated on-demand.