Add a "tracing visitor" to make it easier to find decode issues

[jsdw]

The main motivation for this is in making it easier to understand what went wrong when we try to decode historic blocks ie via https://github.com/jsdw/polkadot-historic-decoding-example.

This visitor is similar to the normal Value visitor, except that it tracks various information while decoding a value, and as such is able to print out a useful error in the event that it fails to decode, ie something like:

Caused by:
    0: Failed to decode type 'Vec<<T as Trait>::Call>' into a Value in extrinsic Utility.batch
    1: Error decoding value at .[1].[0].proof.[69]: Not enough data to fill buffer
       Decoded so far:
       
       <Vec<<T as Trait>::Call>> (
         <<T as Trait>::Call> Staking (
           <builtin::module::Staking> bond {
             controller: <<T::Lookup as StaticLookup>::Source> 0x1650C532ED1A8641E8922AA24ADE0FF411D03EDD9ED1C6B7FE42F1A801CEE37C,
             value: <BalanceOf<T>> 50000000000000000,
             payee: <RewardDestination> Controller ()
           }
         ),
         <<T as Trait>::Call> Session (
           <builtin::module::Session> set_keys {
             keys: <T::Keys> (
               <AccountId> 0x8BDC9AEC45375D38DD3D88862AF5A3E1EA7512E40AC8CD7CC68DD0E2AB73AFC2,
               <AccountId> 0x9672FF4C25ADFA43D60418A2CEE8839A83CA868D318AFB7E6FBAA0E91BEBF622,
               <AccountId> 0x32EE635C78E4060B9EA8FB3C0914988494B2E2AD63CAAC907AFC964D95689053,
               <AccountId> 0x32B3DBC6A9DDD582ABBFFE31C86F144C4AB67F148B44A58C1821BDD46E4BFC4F
             ),
             proof: <Vec<u8>> 0x56B14C4F72738F9D3FDE8EF8CAFCEA7A83B6EA1A7CCDCFC3F50C73130007040284D717070894C6A4B68FB82C9BECF7F01B603D1112AAB237B07F2955CFDC5F6258F245BB7F
           }
         )
       )

The Value to_string implementation was reworked in the process in order to allow Values to be printed in a non-compact form, so now print!("{value:#}") will print a multiline form and print!("{value}") will continue to print a compact format. The ability to print context and custom format things was also added but is currently not exposed in the APIs; we can figure out what they should look like as a separate PR.

[lexnv]

LGTM!