Consider defining contains behavior more precisely

[springcomp]

See discussion in #102.

The contains() function seems to have a well defined behavior when its first argument is an array, as it is based upon equality with one of the array element and equality is well defined in JMESPath.

However, contains() is not sufficiently defined for string arguments. I think a new JEP is warranted to close the loop and tighten its behavior to only accept a string search argument and define containment by the intuitive occurence of an equal substring within the subject string.

[springcomp]

Also, some implementations process arguments differently:

• contains('abc', `true`) -> false
• contains(`abc`, `true`) -> invalid-type

Some implementations deal with types differently within the same implementation:

• contains('abc', `true`) -> invalid-type
• contains(['abc'], `true`) -> false

The JEP must remove this ambiguity.

[springcomp]

#159