Dup and swap Operations

[Bhargavasomu]

What was wrong?

DUP and SWAP EVM operations aren't implemented
Related to Issue #60

How was it fixed?

Following frontier yellowpaper

Cute Animal Picture

[Bhargavasomu]

This PR is an extension of #59 . Please review post the first commit of this PR. This is to be merged post #59

[codecov-commenter]

Codecov Report

Merging #62 (9fcf071) into pyspec-next (dfc5646) will increase coverage by 0.86%.
The diff coverage is 98.96%.

@@               Coverage Diff               @@
##           pyspec-next      #62      +/-   ##
===============================================
+ Coverage        94.57%   95.43%   +0.86%     
===============================================
  Files               13       14       +1     
  Lines              793     1096     +303     
===============================================
+ Hits               750     1046     +296     
- Misses              43       50       +7     

Flag	Coverage Δ
unittests	95.43% <98.96%> (+0.86%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/ethereum/utils.py	83.33% <83.33%> (ø)
src/ethereum/vm/instructions.py	98.49% <98.88%> (+9.60%)	⬆️
src/ethereum/base_types.py	93.37% <100.00%> (+0.21%)	⬆️
src/ethereum/vm/__init__.py	100.00% <100.00%> (ø)
src/ethereum/vm/gas.py	92.30% <100.00%> (+2.30%)	⬆️
src/ethereum/vm/interpreter.py	100.00% <100.00%> (ø)
src/ethereum/vm/ops.py	100.00% <100.00%> (ø)
src/ethereum/spec.py	85.47% <0.00%> (-1.28%)	⬇️

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