add accessors for the underlying stat data

[cehteh]

This anticipates the statx() interface I'll add soon.

Breaking change: Adds a 'SimpleType::Unknown' which can be returned when the filetype can't be
determinded. Code matching on SimpleType may add a clause for this when there is no catch-all.

[cehteh]

The above adds interfaces to the low level stat data as Option<>. The return types are
(mostly) from what the Linux statx() provides. My plan is to make this portable for now (fix
the integration tests). Later add a 'statx' as feature and use statx() instead stat() when
enabled. With statx() one can do some optimizations like querying only parts of the stat data
or build the metadata structure on demand when queried.

[cehteh]

Checks passed, but not extensively tested now. I will use and test that in my project now. The 'statx()' implementation will be postponed until this API has be stabilized.