For expediency, the initial implementation of linear types in GHC
made it so that let and where binders would always be considered
unrestricted. This was rather unpleasant, and probably a big obstacle
to adoption. At any rate, this was not how the proposal was designed.

This patch fixes this infelicity. It was surprisingly difficult to
build, which explains, in part, why it took so long to materialise.

As of this patch, let or where bindings marked with %1 will be
linear (respectively %p for an arbitrary multiplicity p). Unmarked let
will infer their multiplicity.

Here is a prototypical example of program that used to be rejected and
is accepted with this patch:

```haskell
f :: A %1 -> B
g :: B %1 -> C

h :: A %1 -> C
h x = g y
  where
    y = f x
```

Exceptions:
- Recursive let are unrestricted, as there isn't a clear semantics of
  what a linear recursive binding would be.
- Destructive lets with lazy bindings are unrestricted, as their
  desugaring isn't linear (see also #23461).
- (Strict) destructive lets with inferred polymorphic type are
  unrestricted. Because the desugaring isn't linear (See #18461
  down-thread).

Closes #18461 and #18739

Co-authored-by: @jackohughes