niti: cache anchor_to and some other optimizations

[privat]

Some low-hanging-fruit optimizations in the interpreter. Most are simple caching.

The improvements are impressive for microbenches

$ ./difftime.sh nit ./nit fib.nit 28
+ nit fib.nit 28
317811
User time: 6.19
+ ./nit fib.nit 28
317811
User time: 2.08
Gain: 66.397

But less impressive for more complex programs

$ ./difftime.sh nit ./nit nit.nit ../tests/base_simple3.nit
+ nit nit.nit ../tests/base_simple3.nit
1
2
3
4
5
6
7
8
9
10
User time: 2.42
+ ./nit nit.nit ../tests/base_simple3.nit
1
2
3
4
5
6
7
8
9
10
User time: 1.96
Gain: 19.008

[github-actions]

Test Results

    67 files     337 suites   17m 56s ⏱️
14 236 tests 13 689 ✅ 547 💤 0 ❌
14 627 runs  14 060 ✅ 567 💤 0 ❌

Results for commit f67dd0f.

♻️ This comment has been updated with latest results.

[privat]

I stop the series here. Other changes are more intrusive and might be done in other PRs