This PR implements support for offset constraints in the `grind` tactic.
Several features are still missing, such as constraint propagation and
support for offset equalities, but `grind` can already solve examples
like the following:
```lean
example (a b c : Nat) : a ≤ b → b + 2 ≤ c → a + 1 ≤ c := by
grind
example (a b c : Nat) : a ≤ b → b ≤ c → a ≤ c := by
grind
example (a b c : Nat) : a + 1 ≤ b → b + 1 ≤ c → a + 2 ≤ c := by
grind
example (a b c : Nat) : a + 1 ≤ b → b + 1 ≤ c → a + 1 ≤ c := by
grind
example (a b c : Nat) : a + 1 ≤ b → b ≤ c + 2 → a ≤ c + 1 := by
grind
example (a b c : Nat) : a + 2 ≤ b → b ≤ c + 2 → a ≤ c := by
grind
```
---------
Co-authored-by: Kim Morrison <scott.morrison@gmail.com>
This doesn't completely resolve the danger (only relevant in `prelude`
files) of importing `Init.Data.List.Basic` but not `Init.Data.List.Impl`
and thereby not having `@[csimp]` lemmas installed for some list
operations.
I'm going to address this better while working on `Array`.
