[Fix #40] add parent and ancestor selectors

[Mertzenich]

Description

Implements parent and ancestor selectors, should close #40. These functions act as natural counterparts to both child and descendant:

• child returns the node at the end of a direct chain of direct child relationships, parent returns the node at the start of a direct chain of direct child relationships.
• descendant returns the node at the end of a chain of descendant relationships, ancestor returns the node at the start of a chain of descendant relationships.

Three additional functions were added: compose-unary, parent, and ancestor. The compose-unary function implements the functionality utilized by the latter two functions. I added tests for parent and ancestor.

compose-unary

Takes a unary selector function, such as has-child or has-descendant, and any number of selectors. Each of the selectors are composed together using and and the provided unary selector function. For example, running (compose-unary has-child (tag :div) (class :foo) (attr :disabled)) produces the equivalent of:

(and (tag :div)
     (has-child (and (class :foo)
                     (has-child (and (attr :disabled))))))

parent and ancestor

Given any number of selectors, applies compose-unary to the selectors using has-child and has-descendant respectively, that is:

(apply compose-unary has-child selectors)
(apply compose-unary has-descendant selectors)

Examples

These examples are those contained in the docstrings and tests.

parent

<div><span class="foo"><input disabled></input></span></div>

<div><span class="foo"><b><input disabled></input></b></span></div>

(parent (tag :div) (class :foo) (attr :disabled)) will select the div in the first but not the second.

ancestor

<div><span class="foo"><input disabled></input></span></div>

<div><span class="foo"><b><input disabled></input></b></span></div>

(ancestor (tag :div) (class :foo) (attr :disabled)) will select the div in both.