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

[wjones127]

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

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))

[github-actions]

https://issues.apache.org/jira/browse/ARROW-11342

[emkornfield]

@wjones127 sorry for the late review. This looks OK to me, did you want to add specific tests asserting equality to strings?

[wjones127]

@wjones127 sorry for the late review. This looks OK to me, did you want to add specific tests asserting equality to strings?

Yup that makes sense. I will add that.

[pitrou]

@wjones127 Really sorry for the delay :-/. I rebased on git master and will merge if CI is green.