decimal is quadratic

```haskell
Prelude Data.Attoparsec.Text Data.Text> fmap signum (parseOnly decimal (Data.Text.replicate 100000 "9") :: Either String Integer)
Right 1
(0.32 secs, 4,286,165,576 bytes)
Prelude Data.Attoparsec.Text Data.Text> fmap signum (parseOnly decimal (Data.Text.replicate 200000 "9") :: Either String Integer)
Right 1
(1.19 secs, 17,092,657,496 bytes)
Prelude Data.Attoparsec.Text Data.Text> fmap signum (parseOnly decimal (Data.Text.replicate 400000 "9") :: Either String Integer)
Right 1
(4.83 secs, 68,266,651,512 bytes)
Prelude Data.Attoparsec.Text Data.Text> fmap signum (parseOnly decimal (Data.Text.replicate 800000 "9") :: Either String Integer)
Right 1
(18.86 secs, 272,858,846,272 bytes)
```

when doubling the size of input the time quadruples.

Tested using `cabal build --write-ghc-environment-files=always`, and then loading the library into `repl`. So the important parts are not interpreted.

In comparison, `read` is almost instant

```haskell
Prelude Data.Attoparsec.Text Data.Text> signum (read (Prelude.replicate 800000 '9') :: Integer)
1
(0.21 secs, 333,060,168 bytes)
```

Reproduce the scaling measurements in the issue using the `decimal` parser through `parseOnly` in the REPL, then locate the implementation and existing tests for `decimal`. Compare behavior and resource use as the input doubles; done means large integer inputs no longer show quadratic scaling while still returning the same parsed result.

Written by the indexing model from the issue text.