Add IntSliceFieldIndex

[lggomez]

Counterpart of StringSliceFieldIndex, with int values

This PR also includes isolated tests on a new StringSliceFieldIndex indexer (since the previous integration test schema had it inside of a CompoundMultiIndex)

[lggomez]

CI had an unrecoverable error, this shouldn't be related to the changes:

#!/bin/sh -eo pipefail
# No configuration was found in your project. Please refer to https://circleci.com/docs/2.0/ to get started with your configuration.
# 
# -------
# Warning: This configuration was auto-generated to show you the message above.
# Don't rerun this job. Rerunning will have no effect.
false

[jefferai]

Please look at IntFieldIndex and UintFieldIndex, they accept any type/size of uint/int. This makes an assumption as to the value being 32-bit which is much less flexible and potentially unsafe.

[hashicorp-cla]

All committers have signed the CLA.