Re-center perf-numbers for T5631
A little refactoring of HsSplice and friends

Plus adding comments.

The most substantive change is that PendingTcSplice becomes a proper
data type rather than a pair; and PendingRnSplice uses it
Temporarily disable T3064 (see #9771)

This disables T3064 temporarily as it puts a strain on buildbots
during validation exhausting all available memory.
Remove redundant contexts from Foldable methods

New `Foldable` methods accidentally had `Foldable` contexts, which led
to type roles being assigned incorrectly and preventing GND from
deriving `Foldable` instances. Removing those fixes #9761.

Moreover, this patch takes advantage of this fix by deriving
`Foldable` (and `Eq`) for `UniqFM`.

Differential Revision: https://phabricator.haskell.org/D425
Merge branch 'master' into wip/T5462
Use MINIMAL to decide whether we can derive or not, and do not reject newtypes