Added encoding and decoding tests for BigSize

[AdvaitPote]

Fixes #2051

Tests have been added for the test vectors here for both Encoding and Decoding Parts.

[TheBlueMatt]

Thanks!

[TheBlueMatt]

Please squash the commits down into one, otherwise LGTM.

[codecov-commenter]

Codecov Report

Patch coverage: 94.73% and no project coverage change

Comparison is base (c5cc1ed) 91.14% compared to head (c13dca9) 91.14%.

❗ Current head c13dca9 differs from pull request most recent head c593383. Consider uploading reports for the commit c593383 to get more accurate results

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2078   +/-   ##
=======================================
  Coverage   91.14%   91.14%           
=======================================
  Files         101      101           
  Lines       48851    48874   +23     
  Branches    48851    48874   +23     
=======================================
+ Hits        44523    44547   +24     
+ Misses       4328     4327    -1     

Impacted Files	Coverage Δ
lightning-block-sync/src/poll.rs	85.02% <50.00%> (ø)
lightning-invoice/src/lib.rs	80.59% <68.42%> (-0.03%)	⬇️
lightning-background-processor/src/lib.rs	78.29% <100.00%> (-0.13%)	⬇️
lightning-block-sync/src/init.rs	92.57% <100.00%> (ø)
lightning-block-sync/src/lib.rs	93.76% <100.00%> (ø)
lightning-block-sync/src/test_utils.rs	92.81% <100.00%> (ø)
lightning-invoice/src/de.rs	75.95% <100.00%> (-0.07%)	⬇️
lightning-invoice/src/payment.rs	80.76% <100.00%> (+0.76%)	⬆️
lightning-invoice/src/ser.rs	91.59% <100.00%> (ø)
lightning-invoice/src/utils.rs	96.90% <100.00%> (ø)
... and 11 more

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[AdvaitPote]

Apologies for the initial mixup. There was a bit of a confusion. I had accidentally picked some commits in my branch. I have dropped them and It's sorted now!

[TheBlueMatt]

Doesn't bother to correct that these match the upstream test case but either way it's fine.

[AdvaitPote]

Thank you
Should I go ahead with the proposed changes now @TheBlueMatt @wpaulino

[TheBlueMatt]

Yes, please address wilmer's feedback. For a simple PR like this feel free to just squash the fixups as you go rather than adding new commits.