Add "readwire" interface to *some* Graphene Types.

[jhtitor]

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This is useful when you want to read the serialized wire protocol back into objects-in-memory. Although this is rarely used on the client side of graphene networks, at least one particular use case (BitShares blind transfers) exists.

To clarify: this is __bytes__ method in reverse.

The _readwire interface and method signature are pretty horrid, so some redesign might be required.

[codecov-io]

Codecov Report

Merging #65 into develop will decrease coverage by 0.34%. The diff coverage is 87.3%.

@@             Coverage Diff             @@
##           develop      #65      +/-   ##
===========================================
- Coverage    98.69%   98.34%   -0.35%     
===========================================
  Files           24       24              
  Lines         1452     1515      +63     
  Branches       130      135       +5     
===========================================
+ Hits          1433     1490      +57     
- Misses          16       19       +3     
- Partials         3        6       +3

Impacted Files	Coverage Δ
graphenebase/account.py	97.77% <100%> (+0.05%)	:arrow_up:
graphenebase/types.py	96.46% <86.2%> (-2.65%)	:arrow_down:
graphenestorage/masterpassword.py	100% <0%> (+2.43%)	:arrow_up:

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[xeroc]

Would it be possible to add unit tests for those?

[xeroc]

Instead of calling those methods _rewrite, can we have them fromBytes() and not begin with _. That makes it more obvious and corresponds better with the fromBuffer used in bitsharesJS