Accessors for SASLib handler object

[tk3369]

When reading a file incrementally, the open function returns a SASLib.Handler object. Honestly, this struct is somewhat heavy and and probably not very well designed. From a data encapsulation perspective, it would be nice to have some accessor functions that returns commonly used data, like those already being displayed from the show function:

julia> h
SASLib.Handler:
  filename:    data_pandas/productsales.sas7bdat
  encoding:    US-ASCII
  platform:    9.0301M2 (32-bit)
  endianness:  LittleEndian
  page size:   8192
  pages:       18
  rows:        1440
  cols:        10

[tk3369]

It's probably better to encapsulate the internal state with a wrapper object with minimal exposed data. With the new metadata function, there's no reason to get meta information from the handler object.

[tk3369]

Probably a better option is to implement properties for the handler object so we can expose useful fields.