We should make the way variables/parameters are acquired
more efficient.
We should eliminate the number of intermediate copies of
the values of variables/parameters.

Currently variables and parameters in XPath expressions are looked
up dynamically during evaluation; the process generates some intermediate
copies of the variable's value.

Example:
<xsl:template match="/">
  <xsl:variable name="var-1" select="'var-value'"/>
  <xsl:value-of select="$var-1"/>
</xsl:template>

The variable is acquired via:

xmlXPathVariableLookup() (xpath.c)
  --> xsltXPathVariableLookup() (libxslt/variables.c)
    --> xsltVariableLookup()
      --> xsltStackLookup()

Later, in xsltVariableLookup(), the XPath object, which is the value
of the variable, is copied with xmlXPathObjectCopy().

Later, in e.g. xsltValueOf(), this object is then converted to a
string object with xmlXPathConvertString(), which creates another
intermediate object (and sorts if a node set); then the string is
aquired via xsltCopyTextString() from the XPath object.

Proposals:

1) Find a way of binding the variables statically to the AST of
   the XPath expression.

2)
  a) Eliminate the copy of the variables value (XPath object).
     This could be done by having flag on the XPath object which
     marks it as immutable. 
  b) Eliminate the intermediate creation of an XPath string object,
     by using xmlXPathCastToString() rather than xmlXPathConvertString().