Why are nested brackets disallowed?

[Matthew Pickering]

There is a check in `RnSplice` which errors on the following program
with nested brackets.

```
prog = [| [| True |] |]

T.hs:4:11: error:
    • Template Haskell brackets cannot be nested (without intervening splices)
    • In the Template Haskell quotation [| True |]
      In the Template Haskell quotation [| [| True |] |]
  |
4 | prog = [| [| True |] |]
  |           ^^^^^^^^^^

```

As far as I can see the check was added in 2013 in this commit,
https://github.com/ghc/ghc/commit/d0d47ba76f8f0501cf3c4966bc83966ab38cac27#diff-58794f81a415f148d011de563988c2b5R170

But there is no note, no tests and no comment about why it was added.

I removed the check and added a `BracketE` constructor to the
template-haskell AST and the
code compiles fine.

I can also construct a program which needs to be spliced twice and
this also works fine.

```
func Add = [| (+) |]
func Mul = [| (*) |]

f1 "+" = [| Add |]
f1 "*" = [| Mul |]

comb s = [| func $(f1 s) |]
```
```
res = $($(comb "*"))
```

So it seems the restriction is quite arbitrary but I was wondering if
I was missing some limitation which meant this check was added. I
would not be surprised if something more complicated goes wrong with
splicing.

Cheers,

Matt