Extend supported commands

[sushant94]

smtlib.rs currently only supports a few of the defined standard command.
According to the SMTLIB2 standard the following functions are to be recognized and implemented for every solver:

• set-logic
• define-fun
• declare-fun
• declare-sort
• define-sort
• assert
• get-assertions
• check-sat
• get-proof
• get-unsat-core
• get-value
• get-assignment
• push
• pop
• get-option
• set-option
• get-info
• set-info
• exit

We would like to support them all