Use parse(T, str) over constructors

[rofinn]

Currently, parse(T, str) basically defaults to T(str) which doesn't seem ideal. Especially with #72, we may want to use parse more extensively and reserve constructors for explicit type construction.

Base on the julia docs this seems like what we want:

• Constructors are explicit, but may manipulate values as necessary.
• convert should be lossless (e.g., convert(String, convert(::PathType, str))) == str)
• parse/tryparse can be lossy, but are explicitly for parsing string representations of paths)

In theory, if you don't expect anyone to explicitly construct your type and folks are just relying on internal default behaviour of join, Path, p"" then a new type could probably just use the default constructor and only implement parse(T, str).