This adds type and struct field context to error messages.
Instead of "hex string of odd length" users will now see "json: cannot
unmarshal hex string of odd length into Go struct field SendTxArgs.from
of type common.Address".
This commit makes the wrapper types more generally applicable.
encoding.TextMarshaler is supported by most codec implementations (e.g.
for yaml).
The tests now ensure that package json actually recognizes the custom
marshaler implementation irrespective of how it is implemented.
The Uint type has new tests, too. These are tricky because uint size
depends on the CPU word size. Turns out that there was one incorrect
case where decoding returned ErrUint64Range instead of ErrUintRange.
