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.
