Consider adding `trace_amount` as a native host function.

[sappenin]

As of #79, tracing an amount involves a helper function defined in craft (see trace_amount in trace.rs) which merely delegates to trace_num depending on the TokenType.

Instead, we should consider defining a trace_amount host function so that rippled can control the tracing, and emit better output.

[mvadari]

If we do this, we should also add a trace_account host function for AccountIDs

[pwang200]

No objections.

[sappenin]

Note: Another reason not to simply delegate token tracing to trace_num is that trace_num takes an i64, but to support MPT, we need a u64.

If we don't want to introduce a special host function for tracing Tokens, then we should consider switching trace_num to accept a byte array instead of a primitive number (and just have sugar functions that accept a u64).

[mvadari]

Note: I'm already most of the way through implementing both of these in rippled (just working on tests)

[mvadari]

XRPLF/rippled#5674