Improvement for record deserialization/serialization from/to json

[Bhashinee]

Description:
In the current JSON to record conversion with .constructFrom requires exact matches in field names. Because of this restriction at certain times, we need to manually go through the JSON, retrieve and update the record. It would be good if we can configure the mapping json key for a particular field in the record beforehand and do the conversion using 'constructFrom'. 
For example, in the following record, I need to map JSON's unit_amount field to record's 'unitAmount' field and unit_amount_decimal to 'unitAmountDecimal'. 

Ex: 

public type PriceData record { 
      int? unitAmount?; // `json:"unit_amount"` 
     float? unitAmountDecimal?; // `json:"unit_amount_decimal"`
};

This will be useful when dealing with json payloads coming from third-party servers because some JSON responses have fields with underscores.

[jclark]

This would be useful functionality, which should probably be combined with #239, but it doesn't belong in constructFrom: it should be driven by annotations, so it needs to be a stdlib function not a langlib.

[daneshk]

@jclark I am trying to understand how we could support this as a stdlib function. Yes, we can pass this additional data using annotations, something like,

public type PriceData record {
    @jsonElement { name = "unit_amount" }; 
    int? unitAmount?; // `json:"unit_amount"`

    @jsonElement { name = "unit_amount_decimal" };
    float? unitAmountDecimal?; // `json:"unit_amount_decimal"`
};

Is the stdlib function, a util function which accepts record type[may be we can infer it from return type] and JSON value as input params and returns an instance of the record with populated values. something like,

PriceData priceData = check jsonutils:toAnyData(jsonValue, type = PriceData);

json value = check jsonutils:fromAnyData(priceData)

We are also trying to get rid of jsonutils, xmlutils, stringutils modules. I wonder where we should place these functions