Simple string interpolation.

This allows "%(...)" inside a string literal to interpolate the
stringified result of an expression.

It doesn't support custom interpolators or format strings, but we can
consider extending that later.

test/language/module/cyclic_import/a.wren

@@ -2,8 +2,8 @@
 System.print("start a")
 
 var A = "a value"
-System.print("a defined " + A)
+System.print("a defined %(A)")
 import "b" for B
-System.print("a imported " + B)
+System.print("a imported %(B)")
 
-System.print("end a")
+System.print("end a")

test/language/module/cyclic_import/b.wren

@@ -2,8 +2,8 @@
 System.print("start b")
 
 var B = "b value"
-System.print("b defined " + B)
+System.print("b defined %(B)")
 import "a" for A
-System.print("b imported " + A)
+System.print("b imported %(A)")
 
 System.print("end b")

test/language/module/shared_import/a.wren

@@ -1,5 +1,5 @@
 // nontest
 System.print("a")
 import "shared" for Shared
-var A = "a " + Shared
+var A = "a %(Shared)"
 System.print("a done")

test/language/module/shared_import/b.wren

@@ -1,5 +1,5 @@
 // nontest
 System.print("b")
 import "shared" for Shared
-var B = "b " + Shared
+var B = "b %(Shared)"
 System.print("b done")