Add `valueOf` support to XDR types.

[Shaptic]

Is your feature request related to a problem? Please describe.
It is very convenient to be able to use XDR types that correlate to a native type in an inline, convenient way. For example, you could use Hyper inline with arithmetic operations, e.g.

let increment = new Hyper(1234n) + 1n;

Describe the solution you'd like
valueOf() methods added to each XDR primitive type we support.

Describe alternatives you've considered
Each type has a way to access its raw, underlying value, but this is a little jankier.

Additional context
See @paulbellamy's comment here: stellar/dts-xdr#6 (comment).