Remove jest

The last diff to achieve testing parity, rescript-lang#361, has been merged.

Overall improvements:
- We've gone from 7-9s test time to 1.7-1.9s. This is fast enough that we can now plan to run test as part of the build step. Though right now the build is a few seconds which is too slow for tests to squeeze in. Shame.
- Roundtrip tests went from 50-60s to 25-30s.
- Snapshotting is no longer a dedicated api to learn. Just check the git diff. As I said in rescript-lang#343:
  > The question to ask is "what am I trying to do with snapshot tests?" and the answer for this repo is "I wanna check whether some files changed". After which you realize it's git's job.
- Way cleaner snapshot outputs because no JS and JSON serialization in the way.
- Instead of all these red files, it's a single shell test script of a 100 lines.

Author: chenglou

syntax/README.md

@@ -55,10 +55,7 @@ Run the extended tests:
 make roundtrip-test
 ```
 
-Update jest snapshots:
-```sh
-./node_modules/.bin/jest -u
-```
+Those will tell you whether you've got a test output difference. If it's intentional, check them in.
 
 Debug a file:
 ```sh