Add "null statements" to the compiler

Just a lone semicolon that represents a statement with nothing in it. Allows for suppressing EmptyBlock emissions.

DMCompiler/Compiler/DM/DMAST.cs

@@ -37,6 +37,10 @@ public void VisitObjectVarOverride(DMASTObjectVarOverride objectVarOverride) {
             throw new NotImplementedException();
         }
 
+        public void VisitNullProcStatement(DMASTNullProcStatement nullProcStatement) {
+            throw new NotImplementedException();
+        }
+
         public void VisitProcStatementExpression(DMASTProcStatementExpression statementExpression) {
             throw new NotImplementedException();
         }
@@ -772,6 +776,14 @@ public override void Visit(DMASTVisitor visitor) {
         }
     }
 
+    /// Lone semicolon, analogous to null statements in C.
+    /// Main purpose is to suppress EmptyBlock emissions.
+    public sealed class DMASTNullProcStatement(Location location) : DMASTProcStatement(location) {
+        public override void Visit(DMASTVisitor visitor) {
+            visitor.VisitNullProcStatement(this);
+        }
+    }
+
     public sealed class DMASTProcStatementExpression : DMASTProcStatement {
         public DMASTExpression Expression;
 

DMCompiler/Compiler/DM/DMParser.cs

@@ -194,6 +194,7 @@ public DMASTFile File() {
 
         public DMASTStatement? Statement(bool requireDelimiter = true) {
             var loc = Current().Location;
+
             DMASTPath? path = Path();
             if (path is null)
                 return null;
@@ -618,6 +619,12 @@ public DMASTFile File() {
 
         public DMASTProcStatement? ProcStatement() {
             var loc = Current().Location;
+
+            if (Current().Type == TokenType.DM_Semicolon) { // A lone semicolon creates a "null statement" (like C)
+                // Note that we do not consume the semicolon here
+                return new DMASTNullProcStatement(loc);
+            }
+
             var leadingColon = Check(TokenType.DM_Colon);
 
             DMASTExpression? expression = null;

DMCompiler/DM/Visitors/DMASTSimplifier.cs

@@ -53,6 +53,8 @@ public void VisitProcBlockInner(DMASTProcBlockInner procBlockInner) {
             }
         }
 
+        public void VisitNullProcStatement(DMASTNullProcStatement nullProcStatement) { }
+
         public void VisitProcStatementExpression(DMASTProcStatementExpression statementExpression) {
             SimplifyExpression(ref statementExpression.Expression);
         }

DMCompiler/DM/Visitors/DMProcBuilder.cs

@@ -112,6 +112,7 @@ private void ProcessBlockInner(DMASTProcBlockInner block, bool silenceEmptyBlock
 
         public void ProcessStatement(DMASTProcStatement statement) {
             switch (statement) {
+                case DMASTNullProcStatement: break;
                 case DMASTProcStatementExpression statementExpression: ProcessStatementExpression(statementExpression); break;
                 case DMASTProcStatementContinue statementContinue: ProcessStatementContinue(statementContinue); break;
                 case DMASTProcStatementGoto statementGoto: ProcessStatementGoto(statementGoto); break;