Make safe_url_string safer

[Gallaecio]

Tasks:

• Provide an alternative implementation for safe_url_string based on the URL living standard
• Add missing typing information
• Provide comprehensive test coverage for both implementations, highlighting the differences, and in the process make the new implementation support also RFC 2396 (used by java.net.URI) and RFC 3986 (the previous standard, still popular).
• Ensure the following issues are addressed by the new implementation:
  • Pipe symbol ("|") is not percent encoded #80
  • safe_url_string handling IPv6 URLs #193
• Have the URL living standard parse implementation mostly pass the upstream tests.
• Improve performance
  The new implementation is currently up to 40 times slower than the previous implementation for test cases where both have the same, non-error outcome.
• Clean up the implementation: cleaner code, docstrings…
• Discuss what to do API-wise (deprecate safe_url_string in favor of safe_url? Remove safe_url and make safe_url_string use the new implementation?

[codecov]

Codecov Report

Merging #201 (88f32ae) into master (fb70566) will decrease coverage by 0.99%.
The diff coverage is 94.48%.

@@            Coverage Diff             @@
##           master     #201      +/-   ##
==========================================
- Coverage   95.98%   94.98%   -1.00%     
==========================================
  Files           6       14       +8     
  Lines         473     1377     +904     
  Branches       90      333     +243     
==========================================
+ Hits          454     1308     +854     
- Misses          9       40      +31     
- Partials       10       29      +19     

Impacted Files	Coverage Δ
w3lib/_utr46.py	75.73% <75.73%> (ø)
w3lib/_encoding.py	88.23% <88.23%> (ø)
w3lib/_url.py	98.00% <98.00%> (ø)
w3lib/_infra.py	100.00% <100.00%> (ø)
w3lib/_rfc2396.py	100.00% <100.00%> (ø)
w3lib/_rfc3986.py	100.00% <100.00%> (ø)
w3lib/_rfc5892.py	100.00% <100.00%> (ø)
w3lib/_util.py	100.00% <100.00%> (ø)
w3lib/url.py	98.69% <100.00%> (+0.06%)	⬆️

[Gallaecio]

The new implementation is currently up to 40 times slower than the previous implementation for test cases where both have the same, non-error outcome.

@kmike Shall I look into using Cython here?

[Gallaecio]

I have opened #203 as a simpler approach that is good enough to address #80 and similar issues without any major rework of safe_url_string.

I am closing this pull request, as I am stopping work on it. But I have created #204 so that someone, not necessarily me, can take over and finish the work.