* Move String struct stuff into the utf8string module, from spacy.lang

spacy/utf8string.pxd

@@ -1,5 +1,6 @@
 from preshed.maps cimport PreshMap
 from cymem.cymem cimport Pool
+from murmurhash.mrmr cimport hash64
 
 from .typedefs cimport utf8_t, id_t, hash_t
 
@@ -11,6 +12,18 @@ cdef struct Utf8Str:
     int length
 
 
+cdef struct UniStr:
+    Py_UNICODE* chars
+    size_t n
+    hash_t key
+
+
+cdef inline void slice_unicode(UniStr* s, Py_UNICODE* chars, int start, int end) nogil:
+    s.chars = &chars[start]
+    s.n = end - start
+    s.key = hash64(s.chars, s.n * sizeof(Py_UNICODE), 0)
+
+
 cdef class StringStore:
     cdef Pool mem
     cdef PreshMap _map