Add variable shadowing warning

Some test cases:

```
-- Value shadows value
def addThree (x:Int) : Int =
x = x + 1 -- redefinition of x, warn here
x = x + 2 -- redefinition of x, warn here
x

:p addThree 3
> 6

-- Value shadows type
def shadow (array: x=>Int) : Int =
y:x = 0@x -- use of x as type
x = size x -- redefinition of x as value, warn here
x

:p shadow (iota $ Fin 10)
> 10
```

Motivating use case from @srush: https://www.twitch.tv/videos/833913502?t=01h00m17s

---

Reference Haskell program and compiler diagnostics:

```
module Shadow where

addThree :: Int -> Int
addThree x =
let x = x + 1 in
let x = x + 2 in
x
```
```console
$ ghc -c -Wall Shadow.hs

Shadow.hs:4:10: warning: [-Wunused-matches]
Defined but not used: ‘x’
|
4 | addThree x =
| ^

Shadow.hs:5:7: warning: [-Wname-shadowing]
This binding for ‘x’ shadows the existing binding
bound at Shadow.hs:4:10
|
5 | let x = x + 1 in
| ^

Shadow.hs:5:7: warning: [-Wunused-local-binds]
Defined but not used: ‘x’
|
5 | let x = x + 1 in
| ^

Shadow.hs:5:11: warning: [-Wtype-defaults]
• Defaulting the following constraint to type ‘Integer’
Num a0 arising from a use of ‘+’
• In the expression: x + 1
In an equation for ‘x’: x = x + 1
In the expression: let x = x + 1 in let x = x + 2 in x
|
5 | let x = x + 1 in
| ^^^^^

Shadow.hs:6:7: warning: [-Wname-shadowing]
This binding for ‘x’ shadows the existing binding
bound at Shadow.hs:5:7
|
6 | let x = x + 2 in
| ^
```

No file or test entry point is named. Start by locating the compiler's handling of value and type bindings, then compare its diagnostics with the addThree and shadow examples; done means those redefinitions produce warnings at the indicated locations.

Written by the indexing model from the issue text.