RFC 56: Make buffered.reader.line return String iso^

[mfelsche]

This implements RFC 56 and fixes #2755

This is a breaking change for code bases that do not use explicit types when assigning the returned String iso^ to a variable:

let line = reader.line()?

These code bases need to either add an explicit type to their variable (let line: String val = reader.line()?) or need to consume the line upon aliasing or giving away to another function/behaviour.

[SeanTAllen]

@mfelsche is this a breaking change? Given that folks have to consume. It seems like it is. In which case I'd like to wait til after 0.22.6 is released to merge. Putting "DO NOT MERGE" label for now.

[mfelsche]

Release notes draft:

buffered.Reader.line() has been changed to return a String iso^ instead of a String val. This is more powerful as it allows you to mutate the lines returned in place, which was impossible with String val. Nonetheless this is a breaking change. It will break code that relies on type inference, avoiding stating the type for the variable the line is assigned to:

let line = reader.line()?
process_line(line)

These code bases need to adapt in one of two ways:

• mark the line variable explicitly as String val:

  let line: String val = reader.line()?
  process_line(line)

• consume the variable upon usage, as it is iso now:

  let line = reader.line()?
  process_line(consume line)