Handles scopes logic.

Scopes are case-sensitive strings without any whitespace, that represent
authorization access. From OAuth2 RFC (https://tools.ietf.org/html/rfc6749#section-3.3):

> The value of the scope parameter is expressed as a list of space-
> delimited, case-sensitive strings.  The strings are defined by the
> authorization server.  If the value contains multiple space-delimited
> strings, their order does not matter, and each string adds an
> additional access range to the requested scope.
>
>   scope       = scope-token *( SP scope-token )
>   scope-token = 1*( %x21 / %x23-5B / %x5D-7E )

In order to manage fine-grained authorizations, this lib uses a convention
for scope formats.
For example, we often need to distinguish between a full scope that will provides
full access to some resource, and read-only access.
Sometimes we also want to limit the access to some sub-resource.
Here are some examples of our convention:

`users`                      full access to users resource
`users/profile`              access to users profile only
`users/profile:read`         access to users profile read-only
`users/profile/email:write`  access to users profile only email write-only

Mainly `:` is only authorized to split between access read/write/rw
(nothing implies rw).

Sub-resources can be separated by `/`.

This library provides helper functions to check that
a given scope will also grant e.g. `users/profile/email` and `users/profile:read`.

We also provide helpers to normalize sets of scopes:

>>> (normalize-scopes #{"users" "users/profile/email:read" "admin"})
#{"users" "admin"}

...as `users/profile/email:read` is redundant, it is removed.

Note that scopes are meant to be used in an OAuth2 access in mind, and thus
are generally manipulated as a set of scopes.

scopes that do not have any subpath are called _root scopes_.

This is important because it is easy to add, union scopes.
But it is generally impossible to remove just a sub-scope as it would
mean we should know all the sub-paths of some root-scope and add the difference.

Scope are additive by nature.