Allow dict patterns in let bindings

[chris-martin]

This seems rather equivalent to putting them in lambda parameters, and I don't think introduces any syntactic ambiguity, so I see no reason not to add this.

[chris-martin]

We should rename Param to Pattern while we're at it, and explain in the documentation that a single-variable binding is just a simple special case of a pattern.

[chris-martin]

Patterns would be strictly more general than the inherit style of let binding.

let { a, b, ... } = c; in d would be equivalent to let inherit (c) a b; in d

But these expressions have no direct analog in Nix:

• Patterns with defaults: let { a, b ? e, ... } = c; in d
• Patterns without the ellipsis: let { a, b } = c; in d

[chris-martin]

I'm not sure now whether inherit should be allowed for let bindings. I'd like to remove it for simplicity. However, it does have an appealing symmetry with inherit in dict literals. So that also raises the question, then, of whether the dict version of inherit can also be obviated in some way. I don't see any good way to do that, though, without introducing a reliance on standard library functions for common use cases. At the moment I am inclined to keep let-inherit around, for the sake of the appealing symmetry.

[chris-martin]

On second thought, dict bindings that inherit from another dict could work just the same way.

bricks-repl> let x = { a = "1"; b = "2"; c = "3"; }; in { { a, b, ... } = x; }
{ a = "1"; b = "2"; }