ratago experimental release v0.4

This is a considerably more compliant release. Most stylesheets that stick to core XSLT 1.0 functionality should run without error.

• With one exception, all instructions and built-in functions are now implemented (that exception is xsl:decimal-format and the associated format-number function).
• Roughly two-thirds of the libxslt general test suite are now enabled and passing. There are still a number relating to the core XSLT spec that need to be reviewed, but this is encouraging progress.

An alpha release will happen once decimal-format is implemented. and several known bugs relating to processing of imports have been squashed.

The API is still subject to change, though at this point the main change is likely to involve adding error returns in more places.