Add JSON serialization to big number types

[dukenguyenxyz]

This PR:

• Closes Implement to_json(json : JSON::Builder) on BigDecimal #7856

Summary of Changes:

• Allow serialisation: and .to_json to all Big* structs (BigInt,BigFloat, BigDecimal)

• Serialisation Big*.to_json is enabled through adding JSON::Builder.number methods for each Big* type, serialising into Number types

• Allow deserialisation of BigFloat from Int or Float

• Allow all Big* structs to be used as JSON keys: add .to_json_object_key

• Add missing pending_win32 "does for BigDecimal" to yaml/serialization_spec.cr

Further Discussion:

• This PR seeks to resolve Implement to_json(json : JSON::Builder) on BigDecimal #7856, and is built from the exchanges in Extend Big* classes support for JSON serialization #8070 .

• The reason for opening this PR is because issue Implement to_json(json : JSON::Builder) on BigDecimal #7856 should be resolved swiftly as Big* finds lots of usage in terms of serialisation, yet Extend Big* classes support for JSON serialization #8070 has been open and without activity for almost a year. In this PR, all comments from PR Extend Big* classes support for JSON serialization #8070 are addressed, e.g. disallowing BigInt and BigFloat to be deserialised from a String, allowing Big* to only serialise into Number and not String.

• With regards to RX14's suggestion on building a custom converter to serialize/ deserialize Big* structs into/ from String (Extend Big* classes support for JSON serialization #8070 (comment)), this can be done as a separate PR, or if the maintainers would like this to be addressed here, I would not mind adding it in. However, IMHO, I think the commit in this PR currently is adequate to resolve the original issue, and any other modification can be addressed in a separate PR.

[straight-shoota]

Thank you @dukeraphaelng

[straight-shoota]

Seems now we're just missing the same thing for YAML...