## Description 

This updates the move analyzer to have a notion of a `cursor` and
partially revises autocompletion request handling to use that cursor.
This is just a start, but hopefully paves the way for further and better
autocomplete suggestions in the future.

This PR also makes a few smaller changes:

- `FileByteSize` and `FilePosition` now hold a `FileHash`, not a
`FileId`.
- Added a way to look up a file hash from a file path on `MappedFiles`
- Cleaned up some of the move analyzer's `Symbols` initialization code.
- We now carry `loc` information on `struct` and `enum` definitions
through typing (to determine if the cursor is there).

## Test plan 

- A new test suite for cursors in the analyzer.
- A new test for the autocomplete suggestion of `id: UID` when writing a
struct with `key`, which appeared to be previously untested.

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## Release notes

Check each box that your changes affect. If none of the boxes relate to
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that describes the impact of your changes that a user might notice and
any actions they must take to implement updates.

- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] Indexer: 
- [ ] JSON-RPC: 
- [ ] GraphQL: 
- [ ] CLI: 
- [ ] Rust SDK: