[JEP-3a] Functions - Clarified string data type and containment.

[springcomp]

See discussion/121.

This PR brings the following changes:

    In order to support functions, a type system is needed.  The JSON types are used:

    * number (integers and double-precision floating-point format in JSON)
-   * string
+   * string (a sequence of Unicode code points. Note that a code point is distinct to a code unit)

and:

• boolean contains(array|string $subject, array|object|string|number|boolean $search)

• boolean contains(array|string $subject, any $search)

  
  
  Returns `true` if the given `$subject` contains the provided `$search`

• string.

• value.

  If $subject is an array, this function returns true if one of the elements in the array is equal to the provided $search value.

  If the provided $subject is a string, this function returns true if

• the string contains the provided $search argument.
• there exists within the $subject string at least one equal occurrence
• of the $search string. If the $search value is not a string, the
• function MUST return false.