Add validity tests to cover edge cases and make validation uniform across languages

[willbeason]

Add validity tests to cover edge cases

I added these tests to improve Go coverage for optimizations I want to be sure are safe. Previously only Check() was tested, but not CheckShort(), CheckFull() and Decode(), each of which have their own (previously untested) validation logic. In adding these, I found two cases where the libraries of various language handle validation logic inconsistently. In cases of inconsistency, I defaulted to the behavior used by most language in the repository.

1. Automatically expanding requested short code length if odd. Most of the language implementations automatically add 1 to the length of requested codes if the length indicates it is in the pair part of the code.

2. Validate latitude and longitude bounds within validation logic for full codes. Most of the language implementations only check the first two characters for latitude/longitude bounds when within the validation logic for full codes, not when generally checking if codes are valid. This does introduce a strange case where a code is "valid", but is neither a valid short nor a valid long code. As this was the preexisting behavior, I have defaulted to this.

go: Additionally, I removed the duplicate check for len(code) == 1 in CheckFull(). This path cannot be reached if Check() passes. I've added a test to verify this is the case.

[willbeason]

Opened Issue to document this discrepancy: #751