Define minimal supported version of PHP needed into composer.json

Address hoaproject/Central#70

The core is being splitted into several libraries. `Hoa\Exception` is
one of them.

Converted all leftover mentions of the old "string" repository to "ustring" (documentation only).

First, this patch introduces and testes the `checkIconv` static method
that checks whether the `...

First, in the constructor, we tested whether the `ext/mbstring`
extension was available or not. ...

Quoting https://github.com/php/php-src/blob/7dcfdbbee4/UPGRADING#L378-L387:

> It is no longer p...

In case where the `$char` given to `toCode` is not a valid UTF-8
character, a variable was missi...

Thanks @osaris and @shouze!

The original algorithm uses a combining table for non-spacing characters
but we have `\p{Mn}`, `...

Thanks @nicolas-grekas for the review.

Thanks @jubianchi!

UCS does not support characters above U+FFFF so we use a custom `toCode`
method.

Internally, the direction does not matter. It does not require to
consider the direction to appe...