arrow

ARROW-10209: [Python] Support positional options in compute functions

This makes compute functions easier to use, for example here the required "pattern" option doesn't need to be passed by name:
```
>>> pc.split_pattern("abacab", "a")
<pyarrow.ListScalar: ['', 'b', 'c', 'b']>
```

... and producing the following doc at the prompt:
```
split_pattern(strings, /, pattern, *, max_splits=-1, reverse=False, options=None, memory_pool=None)
    Split string according to separator.

    Split each string according to the exact `pattern` defined in
    SplitPatternOptions.  The output for each string input is a list
    of strings.

    The maximum number of splits and direction of splitting
    (forward, reverse) can optionally be defined in SplitPatternOptions.

    Parameters
    ----------
    strings : Array-like or scalar-like
        Argument to compute function
    pattern : optional
        Parameter for SplitPatternOptions constructor. Either `options`
        or `pattern` can be passed, but not both at the same time.
    max_splits : optional
        Parameter for SplitPatternOptions constructor. Either `options`
        or `max_splits` can be passed, but not both at the same time.
    reverse : optional
        Parameter for SplitPatternOptions constructor. Either `options`
        or `reverse` can be passed, but not both at the same time.
    options : pyarrow.compute.SplitPatternOptions, optional
        Parameters altering compute function semantics.
    memory_pool : pyarrow.MemoryPool, optional
        If not passed, will allocate memory from the default memory pool.
```

Closes #11955 from pitrou/ARROW-10209-compute-pos-args

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>