Fix marshal tinyint

[illia-li]

Fixed for tinyint type:

1. string marshal and unmarshal as nullable.
   1.1. Was: marshals "" - caused an error; unmarshals nil data into "0".
   1.2. Now: marshals and unmarshals nil data - "".
2. custom string marshals and unmarshals was not supported, now supported.
3. big.Int marshals was not supported, now supported.
4. Also was fixed cases when marshal and unmarshal should an error, but currently we does not have a corrupt test suite.

Close issues for tinyint:

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

[sylwiaszunejko]

CI keeps failing

[illia-li]

Please don`t merge this PR before #262 and #270

[sylwiaszunejko]

Please clean up the commits

[illia-li]

Please clean up the commits

Done

[sylwiaszunejko]

@illia-li I would like to have the information about what exactly was fixed in the commit message, it will be helpful for future readers to have clearly stated what was wrong before and what this commit changes to make it bette

[illia-li]

@illia-li I would like to have the information about what exactly was fixed in the commit message, it will be helpful for future readers to have clearly stated what was wrong before and what this commit changes to make it bette

Done

[illia-li]

Something wrong with build environment.

[illia-li]

Please, lets not change the test suite in this PR .
This will lead to confusion.
We already have #278 for test suite changes.

[dkropachev]

@sylwiaszunejko , can you please take another look at it.

[sylwiaszunejko]

LGTM

[dkropachev]

@illia-li , could you please rebase

[illia-li]

@illia-li , could you please rebase

Done