Ported mortal era encoding from its Rust implementation

[lyk0rian]

I ported the mortal era encoding from its Rust implementation, which can be found there
https://github.com/paritytech/substrate/blob/c2ec5bc8f12bb5a084b976f2dc1280796e9c1b23/primitives/runtime/src/generic/era.rs#L66

Mortal era encoding can be tricky. What a good thing that we now have a GOlang implementation.

[codecov]

Codecov Report

Merging #172 (9dfe2c1) into master (4fec3b3) will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #172      +/-   ##
==========================================
+ Coverage   58.49%   58.61%   +0.11%     
==========================================
  Files          98       98              
  Lines        3602     3612      +10     
==========================================
+ Hits         2107     2117      +10     
  Misses       1081     1081              
  Partials      414      414              

Impacted Files	Coverage Δ
types/extrinsic_era.go	74.19% <100.00%> (+12.28%)	⬆️

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[vedhavyas]

@lyk0rian this is great! Could you also add an example on how Mortal era could be added to extrinsic instead of immortal and verify that works and also add a failing case ?

The changes looks good on first view. Will take another look again soon. Thanks!