Implement missing SMT-Commands

[laurenzlevi]

Implement all missing SMT language commands:

• reset
• push/pop #50
• reset-assertions
• define-sort
• define-fun #56
• defines-fun(s)-rec
• declare-datatype(s)
• get-assertions
• check-sat-assuming/get-unsat-assumptions
• get-assignment/get-value
• get-proof
• get-unsat-core
• get-info/get-option
• echo