Dynamic field are ignored by the typechecker

[yannham]

Describe the bug
Dynamic fields are just entirely ignored by the typechecker, which means we can write unsound code that will typecheck but fail at run time (including failing unexpectedly late because contracts issued from static types have been elided).

To Reproduce

$ nickel repl
> :tc (let x = 1 in {"%{x}" = 1, bar = false}) : {_ : Bool}
Ok: { _ : Bool }

Expected behavior
I expect that the typechecker correctly ensures that a record with dynamic fields is inferred/checker to be of type {_ : T} for some T and that the fields defined dynamically are actually checked against this type.

Additionally, I expect that the interpolated expressions to be checked to be of type String.

Environment

• Version of the code: eaf676c

Additional context
I realized this as part of #2121. It's clear from the code that we don't visit dynamic fields at all. This is expected to be fixed by #2121, so not much to do on that front until then.