XQuery: Faster EBV checks

[ChristianGruen]

In various cases, EBV checks can be made faster if the tested result is not materialized. Examples:

let $count := 1 to 10000000000
return (
  boolean(concat($count))
  boolean(string-join($count))
  boolean(codepoints-to-string(($count) ! 0x41))
)

Other expressions that could be optimized:

• for strings: fn:normalize-space
• for nodes: paths & steps