TextEdit & Highlighting: performance improvements & changes to architecture

I ran pprof to find what was causing stuttering, and found it to be the
getTabCountInLineAtCol function in TextEdit, because it was iterating
many bytes of the buffer, for each rune rendered. Replaced it with a more
optimal system. Also changed the architecture of the highlighting system
to use a single RegexpRange structure for all regular expressions. This
allows for optimizations and multiline matches in the future.

ui/buffer/rope.go

@@ -96,7 +96,11 @@ func (b *RopeBuffer) Line(line int) []byte {
 // inclusive bounds. The returned value may or may not be a copy of the data,
 // so do not write to it.
 func (b *RopeBuffer) Slice(startLine, startCol, endLine, endCol int) []byte {
-	return (*rope.Node)(b).Slice(b.pos(startLine, startCol), b.pos(endLine, endCol)+1)
+	endPos := b.pos(endLine, endCol)+1
+	if length := (*rope.Node)(b).Len(); endPos >= length {
+		endPos = length-1
+	}
+	return (*rope.Node)(b).Slice(b.pos(startLine, startCol), endPos)
 }
 
 // Bytes returns all of the bytes in the buffer. This function is very likely