Summary: Adds `@functools.lru_cache` to `ts_to_dt` and `ts_to_date` which are used in `_make_rich_value` in Date/DateTime columns, i.e. every time such a column is accessed in a formula. I noticed that these operations are surprisingly expensive while working on https://phab.getgrist.com/D4075. This is just an easy way to potentially significantly speed up certain docs and formulas.
Test Plan:
Put this code in an engine test case:
```
def test_(self):
self.apply_user_action(["AddTable", "Table1", [
{"id": "A", "type": "DateTime:America/Chicago"},
]])
self.add_records("Table1", ["A"], [
[i] for i in range(1000)
])
formula = "for _ in range(1000): $A\nreturn 1"
import time
start = time.time()
self.add_column(
"Table1", formula, type="Any", isFormula=True, formula=formula
)
elapsed = time.time() - start
print(f"Took {elapsed:.2f} for formula {formula}")
```
The time goes from ~10s to ~1s. Similar tests showed no noticeable slowdown when caching had no effect.
Reviewers: dsagal
Reviewed By: dsagal
Differential Revision: https://phab.getgrist.com/D4099
