Avoid allocations in Ability class.

It won't change anything after they are first invoke, so add method cache to avoid allocations and avoid GC.

Benchmarks:

```
Calculating -------------------------------------
project_guest_rules without method cache
                        79.352k i/100ms
project_guest_rules with method cache
                        93.634k i/100ms
-------------------------------------------------
project_guest_rules without method cache
                          2.865M (±32.5%) i/s -     11.982M
project_guest_rules with method cache
                          4.419M (± 7.4%) i/s -     22.004M

Comparison:
project_guest_rules with method cache:  4418908.0 i/s
project_guest_rules without method cache:  2864514.0 i/s - 1.54x slower

Calculating -------------------------------------
project_report_rules without method cache
                        53.126k i/100ms
project_report_rules with method cache
                        97.473k i/100ms
-------------------------------------------------
project_report_rules without method cache
                          1.093M (±36.5%) i/s -      4.675M
project_report_rules with method cache
                          4.420M (± 7.2%) i/s -     22.029M
Comparison:
project_report_rules with method cache:  4420054.3 i/s
project_report_rules without method cache:  1092509.6 i/s - 4.05x slower
```

https://gist.github.com/huacnlee/b04788ae6df42fe769e4

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