Decimal support?

It can sometimes be useful to parse numbers as Decimals, e.g. when working with DynamoDB and various other boto3 APIs.

There doesn't seem to be any way to do this during parsing, and doing it after a decode pass can result in lossy behavior that some APIs (such as boto3) don't like.

Start by reviewing the parsing API and the decode pass described in the issue, with attention to how numeric values are currently represented. Define what Decimal support should look like during parsing and verify that values passed to DynamoDB and other boto3 APIs avoid lossy conversion.

Written by the indexing model from the issue text.