More compact LVObject default representation (using Base64url)

[James Powell]

In work on #56, decided to improve the default LVObject serialization. Currently, this is a flattened form converted to JSON string, but this leads to many bytes becoming multi-byte UTF-8 characters, and all the small bytes being in the very-long \uXXXX form (especially the rev numbers on all LVClasses, which have lots of \u0000).

[James Powell]

Investigated a custom “compact” format, but decided to instead use teh more standard base64url. Added identifying characters at front, so we can continue to deserialize from the old format. Used § as lead UTF-8 char (two bytes), followed by another § (acting as a format revision char). Then base64url encoding.

[James Powell]

1.5.3