Implement MeasureSpanner

input/regression/measure-spanner-spacing-pair.ly

Index: input/regression/measure-spanner-spacing-pair.ly
diff --git a/input/regression/measure-spanner-spacing-pair.ly b/input/regression/measure-spanner-spacing-pair.ly
new file mode 100644
index 0000000000000000000000000000000000000000..f920e09203ec47a08aa0661801c83abd2b1f61f9
--- /dev/null
+++ b/input/regression/measure-spanner-spacing-pair.ly
@@ -0,0 +1,32 @@
+\version "2.21"
+
+\header {
+  texidoc="
+The ends of measure spanners may be aligned in various ways.
+"
+}
+
+\new Staff {
+  \key cis \major
+  \tweak text #"2x"
+  \startMeasureSpanner cis''1\stopMeasureSpanner
+  \break
+  \time 3/4
+  \clef bass
+  \tweak text #"100x"
+  \tweak spacing-pair #'(key-signature . clef)
+  \startMeasureSpanner cis2.\stopMeasureSpanner
+  \break
+  \time 4/4
+  \clef treble
+  \tweak text \markup \char ##x221E
+  \tweak spacing-pair #'(time-signature . clef)
+  \startMeasureSpanner cis''1\stopMeasureSpanner
+}
+
+\layout {
+  \context {
+    \Staff
+    \consists "Measure_spanner_engraver"
+  }
+}