Add multiple EncodeTo improvements

[2opremio]

Enum econding:

• Check that enum values are legal before encoding them
• Use e as the receiver name of EncodeTo (it was using s) which is inconsistent with the rest of the code.

Union encoding:

• Error when a supplied discriminant doesn't match a branch ( EncodeTo() should error when encountering invalid union discriminant values #70 )
• When an enum is used as a discrimimnant, check that they are legal before encoding
• Use u as the receiver name of EncodeTo (it was using s) which is inconsistent with the rest of the code.

Layout improvements:

• Factor invocations and error checking. This makes the code more compact and removes error [S1021](https://staticcheck.io/docs/checks#S1021 from the generated code:

-       err = s.SeqNum.EncodeTo(e)
-       if err != nil {
+       if err = s.SeqNum.EncodeTo(e); err != nil {
                return err
        }

• Fix incorrectly nested error checks

        if s.InflationDest != nil {
-               err = (*s.InflationDest).EncodeTo(e)
-       }
-       if err != nil {
-               return err
+               if err = (*s.InflationDest).EncodeTo(e); err != nil {
+                       return err
+               }
        }

Closes #70

[2opremio]

You can see the generated code at stellar/go#4068