selfhost: for-in support for Channel<T>; fix break/continue C codegen

- lib/Channel.bux: add Channel_Recv_Ok<T> for use in for-in desugaring
- hir_lower.bux: Channel for-in desugars to while+recv_ok pattern
- c_backend.bux: emit 'break;' / 'continue;' with semicolon, fix missing
  semicolon when break/continue inside if-block
- bootstrap/hir_lower.nim: skip generic methods in vtable generation to
  avoid referencing non-existent monomorphized C functions

src/c_backend.bux

@@ -438,11 +438,11 @@ func CBE_EmitExpr(cbe: *CEmitter, node: *HirNode) {
 
     // Break / Continue
     if kind == hBreak {
-        StringBuilder_Append(&cbe.sb, "break");
+        StringBuilder_Append(&cbe.sb, "break;");
         return;
     }
     if kind == hContinue {
-        StringBuilder_Append(&cbe.sb, "continue");
+        StringBuilder_Append(&cbe.sb, "continue;");
         return;
     }
 
@@ -609,7 +609,7 @@ func CBE_EmitExpr(cbe: *CEmitter, node: *HirNode) {
             }
             CBE_EmitExpr(cbe, child);
             // Add semicolon for statements that need it
-            if child.kind != hBlock && child.kind != hIf && child.kind != hWhile && child.kind != hLoop {
+            if child.kind != hBlock && child.kind != hIf && child.kind != hWhile && child.kind != hLoop && child.kind != hBreak && child.kind != hContinue {
                 StringBuilder_Append(&cbe.sb, ";");
             }
             StringBuilder_Append(&cbe.sb, "\n");