For lists of size 0, 1 and 2, it avoids the overhead of creating
temporary lists of closures. I measure the overhead with `test1.lean`
and there is no overhead in this case.
`test1.lean` has a test for length = 4, and the overhead is 7%.
We only use longestMatch to implement the Pratt Parser.
The lists should be small. So, the overhead is acceptable.
If it is not. We can add back the `longestMatch` specific for `TermParser`.
cc @kha
The artificial `Thunk` was being used just to make sure a `Parser`
object was small enough to be inlined. We don't need this hack anymore.
Commit 0ea944a adds a new transformation that makes sure the size of
field `info` does not impact the decision on whether field `fn` will be
inlined or not.
