Chameleon

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Bugfixes:

- Fixed an issue where technically invalid input would result in a
compiler error.

Features:

- The markup class now inherits from the unicode string type such that
it's compatible with the string interface.

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Bugfixes:

- The symbol names "convert", "decode" and "translate" are now no
longer set as read-only *compiler internals*. This fixes issue 65.

- Fixed an issue where a macro extension chain nested two levels (a
template uses a macro that extends a macro) would lose the middle
slot definitions if slots were defined nested.

The compiler now throws an error if a nested slot definition is used
outside a macro extension context.

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Features:

- An expression type ``structure:`` is now available which wraps the
expression result as *structure* such that it is not escaped on
insertion, e.g.::

<div id="content">
${structure: context.body}
</div>

This also means that the ``structure`` keyword for ``tal:content``
and ``tal:replace`` now has an alternative spelling via the
expression type ``structure:``.

- The string-based template constructor now accepts encoded input.

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Bugfixes:

- The ``tal:on-error`` statement should catch all exceptions.

- Fixed issue that would prevent escaping of interpolation expression
values appearing in text.

------------------

Bugfixes:

- The ``tal:on-error`` handler should have a ``error`` variable
defined that has the value of the exception thrown.

- The ``tal:on-error`` statement is a substitution statement and
should support the "text" and "structure" insertion methods.

------------------

Bugfixes:

- An encoding specified in the XML document preamble is now read and
used to decode the template input to unicode. This fixes issue 55.

- Encoded expression input on Python 3 is now correctly
decoded. Previously, the string representation output would be
included instead of an actually decoded string.

- Expression result conversion steps are now correctly included in
error handling such that the exception output points to the
expression location.