Implement validation functions

[jpetso]

Sometimes I'm not interested in actually decoding a base-N string into its binary form, I just want to know whether it's valid and then keep it as encoded string.

I was thinking that the current API makes this possible with an empty Result type that makes put() a no-op, one would merely watch for a parse_error being thrown. This could be wrapped in an API like this:

bool <codec>::valid(const T& encoded);
bool <codec>::valid(const [char|uint8_t]* encoded, size_t encoded_size);

It would also be nice if this method didn't throw internally, because that will hit gdb/lldb when listening for arbitrary throws. Given that, the decode() functions should probably expose more overloads that take a std::error_code and set that to an error, instead of throwing. This would also make it possible to use cppcodec entirely without RTTI internals.