arrow

ARROW-11342: [Python] [Gandiva] Expose ToString and result type information

These methods are intended to make it easier to work with and debug the Gandiva expression builder.

```python
import pyarrow
import pyarrow.gandiva as gandiva

builder = gandiva.TreeExprBuilder()

lit = builder.make_literal(1000.0, pyarrow.float64())
print(lit)
# Before: <pyarrow.gandiva.Node object at 0x7f36fd37ecf0>
# After: (const double) 1000 raw(408f400000000000)

field = builder.make_field(pyarrow.field('a', pyarrow.float64()))
print(field)
# Before: <pyarrow.gandiva.Node object at 0x7ff7daf99f90>
# After: (double) a

print(builder.make_function('greater_than', [field, lit], pyarrow.bool_()))
# Before: <pyarrow.gandiva.Node object at 0x7ff7d24bde70>
# After: bool greater_than((double) a, (const double) 1000 raw(408f400000000000))
```

Closes #9288 from wjones127/ARROW-11342/gandiva-repr

Lead-authored-by: Will Jones <will.jones@mscience.com>
Co-authored-by: Will Jones <willjones127@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>