R1:df9c24e75058

Provide some "term vs substring" support for the Ferret engine

Summary:
Ref T12819. Distinguishes between "term" queries and "substring" queries, and tries to match them correctly most of the time. For example:

- `example` matches "example", obviously.
- `~amp` matches "example", but `amp` does not.
- `examples` matches "example" through stemming.
- `"examples"` does not match "example" (quoted text does not stem).
- `"an examp"` does not match "an example" (quoted text is still term text).