Ignore `THIS#TYPE` for duplicate feature error for type features, rename `String.concat1` as `String.concat`

[fridis]

#2792 renamed one String.type.concat as String.type.concat1 to avoid an error due to duplicate features since we would have three versions of the corresponding type features

concat(THIS#TYPE type)
concat(THIS#TYPE type, sep String sep)
concat(THIS#TYPE type, a,b String)

which cause duplicate feature errors since the type parameter THIS#TYPE is optional. However, since the type parameter is implicit anyway and type feature constructors are never called, this error could be suppressed and concat1 could be concat.

[michaellilltokiwa]

also the reason why discard_all can not be named discard in io.buffered currently.

see:

# NYI naming this feature discard leads to error: Duplicate feature declaration
  public discard_all