Before this commit, each `isDefEq u v` invocation would fail if there
were pending universe level constraints. This commit, moves the
postponed universe constraints back to the `MetaM` state.
It also adds the combinator
```lean
withoutPostponingUniverseConstraints x
```
which executes `x` and throws an error if there are pending universe
constraints. We use the combinator at `elabApp` and `elabBinders`.
Without this commit, we would fail to elaborate simple terms such as
```lean
Functor.map Prod.fst (x s)
```
because after elaborating `Prod.fst` and trying to ensure its type
match the expected one, we would be stuck at the universe constraint:
```
u =?= max u ?v
```
Another benefit of the new approach is better error messages. Instead
of getting a mysterious type mismatch constraint, we get a list of
universe contraints the system is stuck at.
cc @Kha
Also, refuse to evaluate an `[init]` decl in the same module (since we don't know whether the initialization is
backtrackable) and always use native symbol of a `[builtinInit]` decl
The following `do` block
```lean
if c_1 then
action_1
else
if cond_2 then
action_2
action_3
```
was being being parsed as
```lean
if c_1 then
action_1
else if cond_2 then
action_2
action_3
```
cc @Kha
We use `MProd` instead of `Prod` to group values when expanding the
`do` notation. `MProd` is a universe monomorphic product.
The motivation is to generate simpler universe constraints in code
that was not written by the user but generated by the `do` macro.
Note that we are not really restricting the macro power since the
`HasBind.bind` combinator already forces values computed by monadic
actions to be in the same universe.
The new test cannot be compiled without this modication.
@Kha the new test may look exoteric, but it reflects an actual
instance in our code base, and the old frontend supports it.
Not sure whether we should keep it or not.
The compiler frontend implemented in C++ is eagerly inlining local
functions. The new test would take an absurd amount of time without
the new hack.
We remove this hack when we re-implement the compiler frontend in Lean.
