Allow arbitrary hashtable increments. Fixes #1742 (#1897)

* Allow arbitrary hashtable increments. Fixes #1742

Right now incrementing some datastructure's values like maps or histograms can
be done with some boilerplate[1] or with `increment` which increments a value
by 1.

This patch allows a second optional parameter to use as the increment.

- [1]:
```
u64 zero = 0, *val;
val = map.lookup_or_init(&key, &zero);
(*val) += inc;
```

Notes:
- Some lines in the documentation where changed because of trailing spaces
deletion
- The test is quite simple right now
- Will update the tools to use `increment` in another PR

* CR changes