Consider size and character validation on FHIRString

Today, this library does not do any validation of strings parsed by it. FHIR defines limitations on size (e.g. 1MB for strings and 64 chars for strings used as `id`) and characters (e.g. Only A..Z, a..z, 0..9 for `id` strings). We should consider if and how such validation can happen in this library.

Start by tracing how FHIRString values are parsed in the library and where the FHIRString model is defined. Review the FHIR rules for string size and character restrictions, especially the distinction between general strings and id strings. Done requires an agreed validation design and tests covering the applicable limits and characters.

Written by the indexing model from the issue text.