Convtools

**Misc**

- updated pipe inlining weights
- added initial set of benchmarks to catch regressions

**Bugfix**

- fixed group by code generation: unnecessary comprehension condition

**Misc**

- simplified inner group by code generation
- updated ``c.group_by``, ``(...).pipe`` and comprehensions to delegate method
calls to their internals where possible
- updated internals of code generation, fixed random seed for reproducible code
generation
- internally replaced ``FilterConversion`` with ``c.iter``
- added internal ``(...).to_iter()`` method (may become public later, once
documented)

**Misc**

- reworked ``c.join`` so it has its custom simpler implementation
(on python 3.9 it has become approximately 1.62x, 1.95x, 2.15x times faster
for inner, left and outer joins correspondingly)

**Features**

- introduced cumulative conversions: ``c.iter(c.cumulative(c.this, c.this + c.PREV))``
- introduced ``c.if_multiple((c.this < 0, c.this * 10), (c.this == 0, None), else_=5)``

Changed `add_label` signature from:
python
(...).add_label(label_name: t.Union[str, dict], conversion)

to:
python
(...).add_label(label_name: t.Union[str, dict])

The reason is that it had confusing behavior of applying the conversion after
labeling.