See PHI1210. For certain large inputs, we spend more time than we need to replacing tabs with spaces. Add some fast paths:

• When a line only has tabs at the beginning of the line, we don't need to do as much work parsing the rest of the line.
• When a line has no unicode characters, we don't need to vectorize it to get the right result.

• Added test coverage.
• Profiled this, got a ~60x performance increase on a 36,000 line 3MB text file.