Go binding: Finish implementation of the String module

[michelchan]

Go binding: Finish implementation of the String module

Item	Detail
Binding	Go
Module	Morphir.SDK.String
Relevant Morphir.IR.SDK module	Morphir.IR.SDK.String
Relevant morphir-jvm code	morphir.sdk.String
Relevant Elm Module	String

Provide a Go lang implementation of the Morphir.SDK.String module for Morphir.SDK.

The implementation should include all the functionality for String as well as tests (we are willing to have this split across PRs).

Please place it here: https://github.com/finos/morphir/tree/features/polyglot-morphir/corelib/go/morphir/sdk/