Add value.* methods to roundtrip between anydata and Ballerina expression-like string

[jclark]

Might be called

function toBString(anydata val) returns string;
function fromBString(string str) returns anydata|error;

[jclark]

Given design of #499, these are in lang.value:

function toBalString(any|error v) returns string = external;
function fromBalString(string v) returns anydata|error = external;

Here fromBalString only works on what toBalString produces for anydata.