Check for distinct declarations in parsing

Issue https://github.com/CakeML/pure/issues/27 ensures that most dead code still gets type-checked. However there are still [problems](https://github.com/CakeML/pure/issues/27#issuecomment-1466223902) with multiple declarations of the same name due to the [`letrec` distinctness pass](https://github.com/CakeML/pure/blob/65f55650c57cd04e5cd36f0aed2c9e59faf00252/compiler/backend/passes/pure_letrec_cexpScript.sml#L105). Also, the parser must still check for closed expressions, even though type inference already does this.

As discussed in a meeting, this issue suggests:
- moving distinct declaration checking to parsing: remove the existing pass and reject programs with multiple declarations of the same name
- removing the parser closedness check, relying on [this result](https://github.com/CakeML/pure/blob/65f55650c57cd04e5cd36f0aed2c9e59faf00252/compiler/backend/passes/proofs/pure_letrecProofScript.sml#L1540) to lift type inference checks upwards

Unfortunately it is not enough to check top-level declarations only, as demand analysis proofs require [`letrec_distinct`](https://github.com/CakeML/pure/blob/8993d77f45afa042f0b5e730b94c1fe7bf2d9876/compiler/backend/passes/proofs/pure_letrecProofScript.sml#L68-L81).

Rejection of programs with multiple declarations of the same name is in line with GHC. However, if we support function-clause pattern-matching in the future, we must be careful to align with GHC's behaviour in warning on overlapping patterns rather than rejecting programs when appropriate.