Starlark: faster dict.keys
Calling `Dict.entries()` to implement `keys` is suboptimal because it:
* wraps with unmodifiable map
* creates `Map.Entry` object for each iteration
This commit also calls `contents.entrySet()` for `items`, but that's
done mostly for consistency.
Test:
```
def test():
for i in range(10):
d = {1: 2, 3: 4}
for j in range(3000000):
d.keys()
test()
```
Result:
```
A: N=17, r=4.813+-0.260
B: N=17, r=4.373+-0.212
B/A: 0.909
```
Closes #12480.
PiperOrigin-RevId: 419645062
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