fix: rename encode/decode + handle padding chars

[saleel]

Problem

• encode() was doing decoding and decode was doing encoding
• during encode, padding chars were incorrectly encoded as 65 (A) instead of = (as both are 0)

Summary

• Renamed encode/decode
• Handle padding chars during encoding
• Add more tests
• Add assertions in input/output size

Apologies for adding too much in one PR

PR Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[saleel]

Note: Usage docs in README.md needs to be updated to align with the changes in this PR once approved/merged.