blocks check that their statements are void

closes #291

This changes the error message "return value ignored" to "expression value is ignored".
This is because this error also applies to {1;}, which has no function calls.

Also fix ignored expression values in std and test.
This caught a bug in debug.readAllocBytes where an early Eof error would have been missed.
See #219.

src/codegen.cpp

@@ -2885,6 +2885,7 @@ static LLVMValueRef ir_render_instruction(CodeGen *g, IrExecutable *executable,
         case IrInstructionIdTestComptime:
         case IrInstructionIdGeneratedCode:
         case IrInstructionIdCheckSwitchProngs:
+        case IrInstructionIdCheckStatementIsVoid:
         case IrInstructionIdTestType:
         case IrInstructionIdTypeName:
         case IrInstructionIdCanImplicitCast: