aws_base64_compute_encoded_len() is now exact, doesn't add 1 extra for null-terminator

[graebm]

Issue:

One of the core principles of aws_byte_buf and aws_byte_cursor, is you DO NOT assume there's a null-terminator, because null-terminators, and the resulting confusion between string "length" and "size" have led to so many bugs in the long history of C.

But aws_base64_encode() tried to be "nice" and add a secret null-terminator after .len, but before .capacity. To achieve this, aws_base64_compute_encoded_len() would say it needed 1 more byte than necessary. This caused trouble, catching Dmitriy off guard the other day. Searching for uses, I see aws-crt-cpp also once had a bug due to this (fixed in this PR)

In changing this, I found that aws_hex_encode() did similar, but even worse. It always added a null-terminator, and included the null-terminator in the .len! Bad! Fortunately, this function was never used except in generating random data for tests. When Bret needed a function like this in 2019 for signing, he avoided this function entirely and built his own alternate version that doesn't add a null-terminator (it has other differences too).

Description of changes:

• aws_hex_encode() doesn't add null-terminator anymore
  • aws_hex_compute_encoded_len() updated to account for this
• aws_base64_encode() doesn't add null-terminator anymore
  • aws_base64_compute_encoded_len() updated to account for this
• aws_base64_compute_decoded_len() math adjusted so that overflow is impossible
• document some math that confused me
• stop using the term "size" in tests, we care about "len"

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.21%. Comparing base (6401c83) to head (f1676e8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1188   +/-   ##
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  Coverage   84.21%   84.21%           
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  Files          57       57           
  Lines        5979     5973    -6     
=======================================
- Hits         5035     5030    -5     
+ Misses        944      943    -1     

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