test: improve test cases

[Daanvdplas]

Refactors the the error handling in separate functions for easier testing between modules. The tests have also been refactored into separate tests for each error variant and more edge cases. More importantly, the tests don't test the Error from primitives but the raw bytes.

In #192, Error is refactored to the pop api crate, note that the pop primitives crate is removed from the extension crate in this PR since here it was forgotten.

In the second commit docs have been added to the v0 module as well.

Helping with #188

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.32164% with 8 lines in your changes missing coverage. Please review.

Project coverage is 32.76%. Comparing base (6adf1b1) to head (bdfdc96).

Files	Patch %	Lines
extension/src/lib.rs	22.22%	7 Missing ⚠️
extension/src/tests.rs	98.90%	0 Missing and 1 partial ⚠️

@@             Coverage Diff              @@
##           daan/api     #211      +/-   ##
============================================
- Coverage     34.08%   32.76%   -1.33%     
============================================
  Files            34       34              
  Lines          3013     2994      -19     
  Branches       3013     2994      -19     
============================================
- Hits           1027      981      -46     
- Misses         1954     1980      +26     
- Partials         32       33       +1     

Files	Coverage Δ
extension/src/v0.rs	100.00% <100.00%> (ø)
extension/src/tests.rs	92.42% <98.90%> (+4.39%)	⬆️
extension/src/lib.rs	10.59% <22.22%> (+2.48%)	⬆️

... and 1 file with indirect coverage changes

[chungquantin]

Should we assign this to variables? Similar to extract_env. Maybe

let (error_indx, error_type, ..double_nested_error) = encoded_error;

Or we can simply assign encoded_error[1..] to one variable because it is used in multiple places

[Daanvdplas]

No longer needed due to #218