Add function compare blocks

[GregTheGreek]

Description

I'm still adding more tests, but its looking fine as is.

From original PR:

Sometimes it is useful to be able to compare block numbers, which is slightly more complicated than usual because of the option for predefined string block numbers.

Supersedes #3540

Type of change

• New feature (non-breaking change which adds functionality)

Checklist:

• I have selected the correct base branch.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.
• I ran npm run dtslint with success and extended the tests and types if necessary.
• I ran npm run test:unit with success.
• I ran npm run test:cov and my test cases cover all the lines and branches of the added code.
• I ran npm run build and tested dist/web3.min.js in a browser.
• I have tested my code on the live network.
• I have checked the Deploy Preview and it looks correct.
• I have updated the CHANGELOG.md file in the root folder.

[coveralls]

Coverage decreased (-6.3%) to 78.056% when pulling 488135d on greg/compareblocks into 9edd908 on 1.x.