#16 performance improvements

[Christoph Nenning]

We got following tips on the saxon-help mailing list:
- use keys in filter expresions
- use tokenize() instead of str:split
- use functions tnstead of templates
- use atomic values instead of tree fragments
- use if() instead of xsl:choose
- use 'is' instead of = to compare trees/nodes
- declare types in function APIs
- use xsl:sequence instead of xsl:value-of

[Kai K.]

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Added <xsl:strip-space>, now the transformations are 3 times faster. Exchanged <xsl:value-of> where no value value should be returned by a template or function.