Add first class support for converting ABIs themselves to/from json/bin/hex

[allenhan2]

Change Description

As of abieos has lost the implicit ability to convert ABI's themselves between their JSON, binary and hex forms.

Prior to this, the structure of a binary ABI could be defined by an ABI itself resulting in an implicit ability to achieve this use-case by loading the ABI.abi and processing the abi data as it it were transaction/action data.

This task re-adds support for this use-case as explicit first class methods of abieos exposed through the C interface.

These should use the existing conventions from get_bin|hex and to_json functions

This PR is to update:

1. abieos changes for abi conversion
2. move a function to abieos

related PR:
EOSIO/abieos#78

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• Documentation

• Stability bug fix

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Consensus Changes

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• Documentation Additions