Minor compilation speed optimization

src/main/scala/millfork/env/Environment.scala

@@ -662,7 +662,12 @@ class Environment(val parent: Option[Environment], val prefix: String, val cpuFa
 
   def eval(e: Expression, vars: Map[String, Constant]): Option[Constant] = evalImpl(e, Some(vars))
 
-  def eval(e: Expression): Option[Constant] = evalImpl(e, None)
+  def eval(e: Expression): Option[Constant] = {
+    if (e.constantValueCache ne null) return e.constantValueCache
+    val cv = evalImpl(e, None)
+    e.constantValueCache = cv
+    cv
+  }
 
   //noinspection ScalaUnnecessaryParentheses,ZeroIndexToHead
   private def evalImpl(e: Expression, vv: Option[Map[String, Constant]]): Option[Constant] = try{{

src/main/scala/millfork/node/Node.scala

@@ -54,6 +54,7 @@ sealed trait Expression extends Node {
   def #-#(that: Expression): Expression = SumExpression(List(false -> this, true -> that), decimal = false)
 
   @transient var typeCache: Type = _
+  @transient var constantValueCache: Option[Constant] = _
 }
 
 case class ConstantArrayElementExpression(constant: Constant) extends Expression {

src/main/scala/millfork/parser/MfParser.scala

@@ -806,7 +806,7 @@ object MfParser {
   val identifierTail: P[String] =
     CharsWhileIn("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz_.1234567890", min = 1).rep(min = 1, sep = "$").!
 
-  val identifier: P[String] = (letter ~ ("$".? ~ identifierTail).?).!.opaque("<identifier>")
+  val identifier: P[String] = (letter ~ ("$".? ~ identifierTail).?).!.map(_.intern()).opaque("<identifier>")
 
   val doubleQuotedString: P[String] = P("\"" ~/ CharsWhile(c => c != '\"' && c != '\n' && c != '\r').?.! ~ "\"")