Enable --records-strict by default for {json,yaml}-to-dhall

[Gabriella439]

This adds a new --records-loose flag and changes the default behavior
to --records-strict

The rationale for this change is so that this default behavior helps
more when users are still experimenting with the appropriate type to
use to decode a large and messy YAML file.

For example, suppose a user is trying to translate the following YAML
to Dhall:

foo: 1
bar: true

... and they start with a empty Dhall type which they plan to fix in
response to error messages. They'd get a surprising success:

$ yaml-to-dhall '{}' < ./example.yaml
{=}

... because the old default behavior is to ignore extra fields in the
YAML. However, if we're strict by default then users don't have to
apply the --records-strict flag to get feedback on what they need
to change.

[sjakobi]

I was wondering what the error message would look like:

$ stack exec -- yaml-to-dhall '{}' < example.yaml 


Error: Key(s) foo, bar present in the YAML object but not in the expected Dhall record type. This is not allowed unless you enable the --records-loose flag:

Expected Dhall type:
{}

YAML:
foo: 1
bar: true

I noticed that the message Error: Key(s) … would get longer and longer with each key. It might be better to list the unexpected keys after the error message.

We probably also shouldn't include the full input file in the error message.

👍 for the new default though!

[Gabriella439]

@sjakobi: Yeah, I plan to make further improvements to the error message here. I'm just fixing one issue at a time