compiler: do not panic on duplicate functions

Instead, show a regular error message. This is much more user-friendly.

compiler/compiler.go

@@ -826,7 +826,8 @@ func (c *Compiler) parseFunc(frame *Frame) {
 		fmt.Printf("\nfunc %s:\n", frame.fn.Function)
 	}
 	if !frame.fn.LLVMFn.IsDeclaration() {
-		panic("function is already defined: " + frame.fn.LLVMFn.Name())
+		c.addError(frame.fn.Pos(), "function is already defined:"+frame.fn.LLVMFn.Name())
+		return
 	}
 	if !frame.fn.IsExported() {
 		frame.fn.LLVMFn.SetLinkage(llvm.InternalLinkage)