Seq: make whole/part-container transforms such as fmap strict

[infinity0]

As described in the latest documentation. Possible candidates beyond fmap:

• scan*
• update, insertAt, and anything else we might've missed with a similar type signature -> a .. -> (Seq a, *).
• dynamic constructors e.g. from*, replicate*, iterateN. although the documentation says they are O(n), from a quick reading of the source code it seems this really means "only if someone actually evaluates the whole thing e.g. by printing it", since the lazy parts of the Deep structure stay lazy

Some technical details in haskell/containers#752

[infinity0]

By sheer luck, in many cases giving a undefined will crash the dynamic constructors, because the periphery of the data structure is forced in the data definition. However we can carefully hack around this luck like this:

ghci> import Data.Strict.Sequence as S
ghci> uf = [1, 1, 1, 1, 1, undefined, 1, 1, 1, 1]
ghci> S.length $ S.fromList uf
10

The last would be expected to fail in a proper strict container, and it does with Vector:

ghci> import Data.Strict.Vector as V
ghci> V.length $ V.fromList uf
*** Exception: Prelude.undefined
CallStack (from HasCallStack):
  error, called at libraries/base/GHC/Err.hs:75:14 in base:GHC.Err
  undefined, called at <interactive>:14:22 in interactive:Ghci7

[infinity0]

insertAt might be strict-safe by sheer virtue of having to reconstruct one half of the data structure (it crashed for me with undefined, for all params I thought to test), but it's hard to be sure if this holds for all possible parameters; safest to just stick a ! on the param anyway