Fix sc frontend read box

[lassemoldrup]

Purpose

Closes #139.

Changes

• RPC calls could return BigInts, which cannot be serialized with JSON.stringify. This PR uses the package json-bigint as a replacement in places where a return value is serialized to address this.
• Contract method names are fixed in error messages.

Checklist

• [x] My code follows the style of this project.
• [x] The code compiles without warnings.
• [x] I have performed a self-review of the changes.
• [x] I have documented my code, in particular the intent of the hard-to-understand areas.
• [x] (If necessary) I have updated the CHANGELOG.

[DOBEN]

Suggestion: If we have no schema uploaded (and the read box returns the raw bytes), it would be better to return them e.g. in a hex type of string than the Buffer presentation (since for large objects it is hard to copy/read).

[lassemoldrup]

Suggestion: If we have no schema uploaded (and the read box returns the raw bytes), it would be better to return them e.g. in a hex type of string than the Buffer presentation (since for large objects it is hard to copy/read).

I agree, but I feel that there should be a separate issue for this.