nterp: Tweak the add_helper python method.

Rename it to add_slow_path since that is how we use it,
and tweak the arguments to remove lambda from call sites.

Remove the slow-path sharing in iget/iput for simplicity,
debuggabilty, and maintainability.

Cleanup. This has no semantic difference on the code.

Test: diff libart.so binary
Change-Id: Ibcbb5784d53f52e19e413ad4b33a0ef3ca3767e6