pgsql: Convert ExecComputeStoredGenerated to use tuple slots

Convert ExecComputeStoredGenerated to use tuple slots

This code was still using the old style of forming a heap tuple rather
than using tuple slots.  This would be less efficient if a non-heap
access method was used.  And using tuple slots is actually quite a bit
faster when using heap as well.

Also add some test cases for generated columns with null values and
with varlena values.  This lack of coverage was discovered while
working on this patch.

Discussion: https://www.postgresql.org/message-id/flat/20190331025744.ugbsyks7czfcoksd%40alap3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/66a4bad83aaa6613a45a00a488c04427f9969fb4

Modified Files
--------------
src/backend/executor/nodeModifyTable.c  | 33 +++++++++++++++++----------------
src/test/regress/expected/generated.out | 31 +++++++++++++++++++++++++++----
src/test/regress/sql/generated.sql      | 10 ++++++++--
3 files changed, 52 insertions(+), 22 deletions(-)