test($compile): check for memory leaks in nested transclusion
test($compile): check no memory leak with coexisting element transcludes

If an element contains two "element" transcludes then the initial clone
consists of only comment nodes.  The concern was that this meant that
the transclude scopes would not be cleaned up.

But it turns out that in the case that there are only comments then the
scope is never attached to anything so we don't need to worry about cleaning
it up.

Later if a concrete element is created as part of the transclude then these
elements will have destroy handlers.
fix(jqLite): don't attach event handlers to comments or text nodes

We were attaching handlers to comment nodes when setting up bound transclusion
functions. But we don't clean up comments and text nodes when deallocating so
there was a memory leak.

Closes #7913
Closes #7942
test(compile): check transclusion/scopes work with multi-element directives
fix(jqLite): never add to the cache for non-element/document nodes

Calling `jqLite.data()` on a disallowed node type caused an empty object to be added to the
cache. This could lead to memory leaks since we no longer clean up such node types when they are
removed from the DOM.

Closes #7966
fix($parse): handle constants as one-time binding expressions

Handle constant expressions as one-time binding expressions.
Avoids the infinite digest from
https://github.com/angular/angular.js/pull/7960/files#r14136938

Closes #7970
fix(core): drop the toBoolean function

So far Angular have used the toBoolean function to decide if the parsed value
is truthy. The function made more values falsy than regular JavaScript would,
e.g. strings 'f' and 'no' were both treated as falsy. This creates suble bugs
when backend sends a non-empty string with one of these values and something
suddenly hides in the application

Thanks to lgalfaso for test ideas.

BREAKING CHANGE: values 'f', '0', 'false', 'no', 'n', '[]' are no longer
treated as falsy. Only JavaScript falsy values are now treated as falsy by the
expression parser; there are six of them: false, null, undefined, NaN, 0 and "".

Fixes #3969
Fixes #4277