Add new function to get the first grapheme from a string.

Not sure if I have a use-case yet but it seems like it could come in handy.

uniseg/graphemes.scm

@@ -1,5 +1,8 @@
 (define-module (uniseg graphemes)
- #:use-module (ice-9 hash-table)
+  #:use-module (uniseg graphemes stream)
+  #:use-module (ice-9 hash-table)
+  #:use-module (ice-9 streams)
+  #:use-module (srfi srfi-41)
  #:use-module (srfi srfi-1)
  #:use-module (srfi srfi-9 gnu)
  #:export (make-grapheme
@@ -10,7 +13,8 @@
            grapheme-glyphs
            grapheme-glyphs-reverse
            grapheme-state
-           grapheme-string))
+           grapheme-string
+           string->grapheme))
 
 (define-immutable-record-type <grapheme>
   (_make-grapheme width delta-width modification? state glyphs-reverse glyphs-promise string-promise)
@@ -40,3 +44,23 @@
 (define (grapheme-string grapheme)
   "Return a lazily-constructed string of the glyphs in the grapheme."
   (force (_grapheme-string-promise grapheme)))
+
+(define (string->grapheme str)
+  "Reads from `str' until we reach the end of the first grapheme cluster"
+  (define stream (string->grapheme-stream str))
+  (define first-grapheme (stream-car stream))
+
+  ;; Skip first, we want to get the last entry that is a modification
+  ;; (or the first entry, if there are no mods afterwards)
+  (define last-grapheme
+    (let loop ((stream stream))
+      (define grapheme (stream-car stream))
+      (define next-stream (stream-cdr stream))
+      (if (and (stream-pair? next-stream)
+               (grapheme-modification? (stream-car next-stream)))
+          (loop next-stream)
+          grapheme)))
+
+  (if (grapheme-modification? last-grapheme)
+      last-grapheme
+      first-grapheme))