Support `TvmSlice` type of fixed size without references

[IgorKoval]

Proposal:
Support TvmSlice type of fixed size without references (aka bitstring). It can be useful:

• for working with bls.* functions.
• for sending in a contract some binary string of fixed size.

TvmSlice will be stored in the message cell (not in references as it is done for TvmCell).

Example in TVM-Solidity:

function f(TvmSlice384 key, TvmSlice768 signature) public { /* ... */ }