Add function selector check for slow vm

[mininny]

Description

The Go slow implementation is intended to mirror the Solidity implementation. For this, it takes the calldata as input.

However, the slow implementation lacks a check that is abstracted in the Solidity implementation: the function selector.

As the function selector is not checked, calldata with incorrect selector will be executable on the slow implementation but will revert on the Solidity smart contract.

Ensure on the slow implementation that the first 4 bytes of calldata correspond to the correct function selector.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 61.87%. Comparing base (8ff9318) to head (81ee551).

Files with missing lines	Patch %	Lines
rvgo/slow/vm.go	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #108      +/-   ##
==========================================
- Coverage   61.89%   61.87%   -0.03%     
==========================================
  Files          27       27              
  Lines        4091     4094       +3     
==========================================
+ Hits         2532     2533       +1     
- Misses       1427     1428       +1     
- Partials      132      133       +1     

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