(ocaml, piqi server) Fix parsing empty Piq input in frameless mode

Empty input was erroneously throwing Piq.EOF exception.
Add a new .piq-alias property for field and options

This allows to specify an alternative name a field or an option; used only when
parsing Piq; Field names specified via .name or derived from .type take
precedence over names defined using .piq-alias

The ability to use an alternative name is useful when Piq is used as as a
syntax notation for DSLs.