speedup notation3/turtle parser

[white-gecko]

Fixes #1266, #1270 (replaces).
Fixes #1262.
Related to #1261

@rchateauneu, @FlorianLudwig

Proposed Changes

• some micro optimizations
• Replace access to substring of length 1: [i : i + 1] with access to chars [i]
• replace while loop with with find:
  https://github.com/RDFLib/rdflib/compare/master...white-gecko:feature/speedupN3?expand=1#diff-702447d86f727e47363ffbc864d3cef01dbceee6b0652f0e992c71947324dc95L1202
  https://github.com/RDFLib/rdflib/compare/master...white-gecko:feature/speedupN3?expand=1#diff-702447d86f727e47363ffbc864d3cef01dbceee6b0652f0e992c71947324dc95R1201
• move most common case of non-commented line starting with a view spaces to the beginning of skipSpace()
• replace char in "abc" with char in {'a', 'b', 'c'} constructs (set instead of string)

[coveralls]

Coverage increased (+0.1%) to 75.515% when pulling 9653eef on white-gecko:feature/speedupN3 into 9f0f296 on RDFLib:master.

[coveralls]

Coverage increased (+0.1%) to 75.515% when pulling 9653eef on white-gecko:feature/speedupN3 into 9f0f296 on RDFLib:master.

[coveralls]

Coverage increased (+0.1%) to 75.515% when pulling 9653eef on white-gecko:feature/speedupN3 into 9f0f296 on RDFLib:master.

[white-gecko]

The last thing we were looking for are new tests for the changed lines of the parser, but maybe passing the current tests is ok already, and we make a new pr for additional tests. @nicholascar @ashleysommer what do you think?

[white-gecko]

@nicholascar and @ashleysommer: any objections to merge?

[nicholascar]

I'm happy for you to merge.