simpler replacement for <%shared>

The main use of the <%shared> section is to declare variables that can 
be shared between different parts of a component and that persist for a 
single request.

However, <%shared> has some disadvantages:

* It is fragile, often breaking when combined with other features. e.g. 
see bugs 213, 337, 427, 447, 535, and 546, or just search for "%shared" 
on the todo list.

* Its funky implementation - namely, recreating the closures for each 
subcomponent/method for every request - carries an unavoidable 
performance penalty (though this has never been measured).

We can provide almost the same functionality in a simpler, more reliable 
way with a restricted block that simply declares one or more shared 
request variables. e.g.

    <%request_vars>
    $a
    $b => 'default'
    %c
    </%request_vars>

The syntax of this block is familiar from the <%args> section.

Beyond implementation simplicity and efficiency, this section has 
several advantages over <%shared>:

* Mason can automatically cleanup the values of each request var at the 
end of the request. Variables declared in <%shared> cannot be 
automatically cleaned up and thus often look like leaks.

* It is possible to ask a component what its request vars are (similar 
to $comp->declared_args) and even what their current values are; useful 
for debugging and logging.

* It has a clearer name that reminds the user of the lifetime of these 
variables.

Amazon is using a section like this for reasons of efficiency, and 
developers do not seem to miss the more general <%shared> functionality. 
It does not conflict with the <%shared> section - they can coexist - but 
if we introduced this we might consider recommending it and/or slowly 
deprecating <%shared>.

Comments welcome.

Jon




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