Enable module resolution caching to work, even if the file containing the module has changed

[keyboardDrummer]

Currently the Dafny resolution caching will use, as a key for determining whether a module has changed, instead of computing a unique hash for a module, use the unique hash of the file content that the module is in.

This means that if a module is in a file that has changed, there will be no resolution caching for that module.

This is not intuitive, and users that are working with very large files and are suffering from long resolution times, will not think of splitting their project up into smaller files.

[keyboardDrummer]

This requires working differently with tokens. It means that a token should not store absolute file positions, but should contain a TextAnchor that can be resolved to absolute positions using a TextBuffer. The TextBuffer will contain a map from TextAnchor to offsets that it updates when processing document updates.