Enable calling multiple SUT functions per invariant testing run

[BowTiedRadone]

This PR allows multiple public functions from the target contract to be called during an invariant testing run. This way, more state transitions can be caused in fewer runs.

Example Output

₿     1006 Ӿ     1084   wallet_5        counter decrement 
₿     1006 Ӿ     1086   wallet_3        counter add 198869171
₿     1006 Ӿ     1088   wallet_3        counter add 1627479458
₿     1006 Ӿ     1090   wallet_2        counter increment 
₿     1006 Ӿ     1092   wallet_6        counter decrement 
₿     1006 Ӿ     1092   wallet_4 [PASS] counter invariant-counter-gt-zero 
₿     1293 Ӿ     1381   wallet_2        counter decrement 
₿     1293 Ӿ     1383   wallet_1        counter increment 
₿     1293 Ӿ     1385   wallet_1        counter increment 
₿     1293 Ӿ     1387   wallet_3        counter increment 
₿     1293 Ӿ     1389   wallet_2        counter increment 
₿     1293 Ӿ     1391   wallet_3        counter increment 
₿     1293 Ӿ     1393   wallet_6        counter decrement 
₿     1293 Ӿ     1395   deployer        counter add 4
₿     1293 Ӿ     1397   deployer        counter decrement 
₿     1293 Ӿ     1399   deployer        counter increment 
₿     1293 Ӿ     1401   deployer        counter increment 
₿     1293 Ӿ     1403   wallet_3        counter decrement 
₿     1293 Ӿ     1403   wallet_4 [PASS] counter invariant-counter-gt-zero 
₿     1646 Ӿ     1758   wallet_8        counter add 2084438653
₿     1646 Ӿ     1758   wallet_7 [PASS] counter invariant-counter-gt-zero 

This PR resolves #98.

[moodmosaic]

LGTM! Tested also locally via

npm install
npm run build
npx rv example counter test
npx rv example counter invariant
npx rv example cargo test
npx rv example cargo invariant
npx rv example reverse test
npx rv example slice test