Minor: rename variable
#830 "Itemset editor: fixup label/value if needed"
Minor: indent
Minor: indent
Add xxf:custom-mips attribute

This allows explicitly specifying which extension attributes on xf:bind are custom MIPs. If missing, backward-compatible behavior is enabled and any extension attribute is considered a custom MIP.
FB: Set xxf:custom-mips

At design time, we only need to evaluate fb:required. With this change, all other fb:* attributes do not evaluate at design time, which is good for performance with a large number of binds.
Add XPath request statistics for debugging
#834 "MIP evaluations are slow with many bind/@name"

- variables are pulled by MIP expressions as needed
- static MIPs directly point to expressions, without cache indirection
- with XPath analysis enabled, the same compiled expression is used
Fix #835 "Failing constraint should default to false()"
Minor: arrows