Change the name of the VdbeMemRelease() macro to VdbeMemReleaseExtern() to
more accurately reflect what it does.  Performance enhancement to the
sqlite3VdbeMemRelease() function.
Remove the pager_lookup() function since it is redundant with 
sqlite3PagerLookup().
In cases where stat4 data is available but cannot be used because the rhs of a range constraint is too complex a expression, fall back to using the default estimates for number of rows scanned.
Allow CAST expressions and unary "+" operators to be used in the DEFAULT
argument of an ALTER TABLE ADD COLUMN and to be understand on the RHS of
range constraints interpreted by STAT3/4.  This involves a rewrite of the
implementation of the CAST operator.
Test cases added for using unary "+" and CAST operators on the RHS of range
constraints and verifying that STAT3/4 can use those constraints.
Minor changes to the CAST logic to make it more testable.
Add an assert() and five testcase() macros to the OP_Cast opcode implementation
to help verify that it is fully tested.