Add Parser Package

[patrick-ogrady]

Motivation

When parsing operations in a block, it is difficult to group related operations and calculate all balance changes.

Solution

Create parser package that provides support for parsing the balance changes in a block and for grouping related operations.

Bug Fixes

• Remove need to use GO111MODULE=off in Makefile
• Allow Currency with Decimals=0
• Fix panic in BalanceChanges when operation.Amount=nil

Future PR

• Remove balance changes logic from rosetta-cli

[coveralls]

Pull Request Test Coverage Report for Build 1008

• 147 of 166 (88.55%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.7%) to 86.513%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
parser/balance_changes.go	56	65	86.15%
parser/group_operations.go	42	52	80.77%

Totals
Change from base Build 919:	0.7%
Covered Lines:	789
Relevant Lines:	912

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💛 - Coveralls