codec: have lexers return io.EOF when encountered

[Saxon Milton]

Lexers currently return nil when an io.EOF is encountered. This does not seem right… the caller of the lexer should decide what to do with io.EOFs not the lexers.

[Alan Noble]

Agreed. It should be possible for callers to call the shots.

[kortschak]

This needs to be more fully detailed before it's a correct bug.

[Saxon Milton]

Resolved in PR #330