Closes#1483
The error now points to the offending `=`, works correctly with
multi-line clauses, and explains exactly what's wrong. E.g. for
```agda
f : Nat → Nat → Nat;
f zero x = x;
```
we get
```
|
6 | f zero x = x;
| ^
expected ":=" instead of "="
```
A minor disadvantage of the proposed solution is that now it's
impossible to use `=` without parentheses as a top variable name in a
pattern, e.g.
```
f zero = := =;
```
gives an error.
However, if one really wants to name a variable `=`, it is enough just
to enclose it in parentheses:
```agda
f : Nat → Nat → Nat;
f zero (=) := =;
f (suc n) (=) := f n =;
```
I believe this slight non-uniformity is well worth the increased
usability due to a better error message. Confusing `:=` with `=` is very
common. Using `=` as a variable name in a top pattern is rare.
Co-authored-by: janmasrovira <janmasrovira@gmail.com>
* remove ≔ from the language and replace it by :=
* revert accidental changes in juvix input mode
* update stdlib submodule
* rename ℕ by Nat in the tests and examples
* fix shell tests
