text: remove String, Layout and add Glyph

In preparation for using Shaper with an io.Reader, rework the API to not refer
to strings. In particular, introduce Glyph for holding the rune in addition to
the advance. For fast traversing of the underlying text, add Len to Line with
the UTF8 length.

Layout is a useless wrapper around []Line; remove it while we're
here.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

text/lru.go

@@ -20,7 +20,7 @@ type pathCache struct {
 type layoutElem struct {
 	next, prev *layoutElem
 	key        layoutKey
-	layout     *Layout
+	layout     []Line
 }
 
 type path struct {
@@ -42,7 +42,7 @@ type pathKey struct {
 
 const maxSize = 1000
 
-func (l *layoutCache) Get(k layoutKey) (*Layout, bool) {
+func (l *layoutCache) Get(k layoutKey) ([]Line, bool) {
 	if lt, ok := l.m[k]; ok {
 		l.remove(lt)
 		l.insert(lt)
@@ -51,7 +51,7 @@ func (l *layoutCache) Get(k layoutKey) (*Layout, bool) {
 	return nil, false
 }
 
-func (l *layoutCache) Put(k layoutKey, lt *Layout) {
+func (l *layoutCache) Put(k layoutKey, lt []Line) {
 	if l.m == nil {
 		l.m = make(map[layoutKey]*layoutElem)
 		l.head = new(layoutElem)