Fix int marshal, unmarshall

[illia-li]

Fixed for int type:

1. string marshal and unmarshal as nullable.
   1.1. Before: marshals "" - caused an error; unmarshals nil data into "0".
   1.2. Now: marshals and unmarshals nil data - "".
2. custom string marshals and unmarshals, before not supported, now supported.
3. big.Int marshals, before not supported, now supported.
4. Marshal data, which equal math.MaxUint32, into uint32, uint64, uint, return error' before, now no error`.
5. Unmarshal data with the length is different from 0 and 4, did not return error before, now it returns `error'.

Close issues for int:

1. Marshal, unmarshal. For many cql types unmarshaling data that have bigger len than cql type, do not return an error. #246
2. Marshal, unmarshal. For many cql types a go string is supported, but not a go custom string #243
3. Marshal, unmarshal. cql types: tinyint, smallint, int can not be marshaled into go big.Int #244
4. Marshal, unmarshal. Problems with zero and null data and values. #250
5. Marshal, unmarshal. For many cql types unmarshaling data that have smaller len than cql type, do not return an error. #252

[sylwiaszunejko]

is this cqlint needed, can you just use int similarly to smallint and tinyint?

[illia-li]

Because identifier int already busy

[sylwiaszunejko]

so maybe rename the package to cqlint to not have this thing here

[sylwiaszunejko]

@dkropachev what do you think? cqlint "github.com/gocql/gocql/marshal/int" is better or "github.com/gocql/gocql/marshal/cqlint"

[dkropachev]

Good idea.

[illia-li]

Done