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/language.go

@@ -1,7 +1,5 @@
 package buffer
 
-import "regexp"
-
 type Syntax uint8
 
 const (
@@ -19,6 +17,6 @@ const (
 type Language struct {
 	Name      string
 	Filetypes []string // .go, .c, etc.
-	Rules     map[*regexp.Regexp]Syntax
+	Rules     map[*RegexpRegion]Syntax
 	// TODO: add other language details
 }