Error checking in decoder

[gavv]

Hello!

Currently there is no error checking in decoder, e.g. in DecodeToInt64():

idx = strings.IndexRune(e.encode, v)
...
c = int64(idx) * ...

strings.IndexRune() could return -1 here.

Would you accept a pull request that adds error checking?

What approach would you prefer?

• panic() on error;
• change return type from int64 to int64, error (this will break API);
• add new function with int64, error return type?