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.

src/vm/wren_core.wren.inc

@@ -133,6 +133,19 @@ static const char* coreModuleSource =
 "class String is Sequence {\n"
 "  bytes { StringByteSequence.new(this) }\n"
 "  codePoints { StringCodePointSequence.new(this) }\n"
+"\n"
+"  static interpolate_(parts) {\n"
+"    var result = \"\"\n"
+"    for (part in parts) {\n"
+"      if (part is String) {\n"
+"        result = result + part\n"
+"      } else {\n"
+"        result = result + part.call().toString\n"
+"      }\n"
+"    }\n"
+"\n"
+"    return result\n"
+"  }\n"
 "}\n"
 "\n"
 "class StringByteSequence is Sequence {\n"
@@ -167,7 +180,7 @@ static const char* coreModuleSource =
 "    return other\n"
 "  }\n"
 "\n"
-"  toString { \"[\" + join(\", \") + \"]\" }\n"
+"  toString { \"[%(join(\", \"))]\" }\n"
 "\n"
 "  +(other) {\n"
 "    var result = this[0..-1]\n"
@@ -189,7 +202,7 @@ static const char* coreModuleSource =
 "    for (key in keys) {\n"
 "      if (!first) result = result + \", \"\n"
 "      first = false\n"
-"      result = result + key.toString + \": \" + this[key].toString\n"
+"      result = result + \"%(key): %(this[key])\"\n"
 "    }\n"
 "\n"
 "    return result + \"}\"\n"