Convtools

**Features**

- introduced `c.date_parse` / `c.datetime_parse` shortcut and extension of
`datetime.strptime`
- introduced `c.date_trunc` / `c.datetime_trunc` to truncate dates to years,
months, days-of-week, etc. (including multiples of them like quarters -
`3mo`). Support offsets.
- introduced `DateGrid` / `DateTimeGrid` helpers to build gap-less series of
dates/datetimes. Support offsets.

**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)``