Html::openElement() micro-optimisations

• Use isset() instead of in_array()

In dropDefaults():

• Remove strtolower(), since the performance cost of running strtolower() on all attribute names far outweighs the performance benefit of dropping default values for attributes with non-canonical capitalisation. Attribute names with non-canonical case are extremely rare in the MediaWiki ecosystem. The change is harmless in the sense that a default attribute is semantically equivalent to an omitted attribute.
• Handle common cases first.
• Avoid value normalization when the attribute name is unknown.
• For class attributes, check if the value is empty using strict comparison rather than normalization.

In expandAttributes():

• If the value is an array, explode each element and add the parts to an array rather than doing implode() then explode().
• If the value is an array, check for spaces in the loop rather than calling array_diff().
• Replace $quote with a literal
• Skip the check for boolean attributes if we already know it is a space-separated attribute.
• Inline Sanitizer::encodeAttribute

Measuring instruction count per iteration with perf stat, averaged over
10M iterations, PS1. Test case:
Html::openElement('a', [ 'class' => [ 'foo', 'bar' ] ] )

• Baseline: 11160.7265433
• in_array(): 10390.3837233
• dropDefaults() changes: 9674.1248824
• expandAttributes() misc: 9248.1947500
• implode/explode and space check: 8318.9800417
• Sanitizer inline: 8021.7371794

Change-Id: I7680b8939da50430316d77d42ebc1987752e99dc