This also removes 'canonicalizePath' from HPath which fixes
the doctest.

Pending upstream PR.

* don't rely on OverloadedString
* make basename fail if run on root dir "/"
* reject "." and ".." in filename parser
* reject ".." in path parsers
* normalise paths in parsers

I wasn't happy with the way it dealt with Dir vs File things. In his
version of the library, a `Path b Dir` always ends with a trailing
path separator and `Path b File` never ends with a trailing path separator.

IMO, it is nonsensical to make a Dir vs File distinction on path level,
although it first seems nice.
Some of the reasons are:
* a path is just that: a path. It is completely disconnected from IO level
  and even if a `Dir`/`File` type theoretically allows us to say "this path
  ought to point to a file", there is literally zero guarantee that it will
  hold true at runtime. So this basically gives a false feeling of a
  type-safe file distinction.
* it's imprecise about Dir vs File distinction, which makes it even worse,
  because a directory is also a file (just not a regular file). Add symlinks
  to that and the confusion is complete.
* it makes the API oddly complicated for use cases where we basically don't
  care (yet) whether something turns out to be a directory or not

Still, it comes also with a few perks:
* it simplifies some functions, because they now have guarantees whether a
  path ends in a trailing path separator or not
* it may be safer for interaction with other library functions, which behave
  differently depending on a trailing path separator (like probably shelly)

Not limited to, but also in order to fix my remarks without breaking any
benefits, I did:
* rename the `Dir`/`File` types to `TPS`/`NoTPS`, so it's clear we are only
  giving information about trailing path separators and not actual file
  types we don't know about yet
* add a `MaybeTPS` type, which does not mess with trailing path separators
  and also gives no guarantees about them... then added `toNoTPS` and
  `toTPS` to allow type-safe conversion
* make some functions accept more general types, so we don't unnecessarily
  force paths with trailing separators for `(</>)` for example... instead
  these functions now examine the paths to still have correct behavior.
  This is really minor overhead. You might say now "but then I can append
  filepath to filepath". Well, as I said... we don't know whether it's a
  "filepath" at all.
* merge `filename` and `dirname` into `basename` and make `parent` be
  `dirname`, so the function names match the name of the POSIX ones,
  which do (almost) the same...
* fix a bug in `basename` (formerly `dirname`) which broke the type
  guarantees
* add a pattern synonym for easier pattern matching without exporting
  the internal Path constructor

Fix a haddock parse error

The line was triggering parse error when running haddock:

```
src/Path.hs:16:1:
    parse error on input ‘-- | A normalizing well-typed path type.’
```

Removing it works for me.

Ping @mrkkrp

Clarify what limitations the RelX parsers have.

Improve description of the package