Add more FunctionDeclarationKinds for the visitor

[daniel-raffler]

Hello,
this PR fixes some minor issues:

• In the MathSAT backend we now properly recognize terms for division. These used to be mapped to FunctionDeclarationKind.OTHER when they should be FunctionDeclarationKind.DIV
• We added support for int_to_bv, ubv_to_int and sbv_to_int. These are need to convert between bitvector and integer terms, and while were able to create such terms with BitvectorFormulaManager.makeBitvector(int length, IntegerFormula pI) and BitvectorFormulaManager.toIntegerFormula) they were not properly supported by the visitor

TODO

• Wait for Add support for indexed functions #507 as int_to_bv is an indexed function and will need to be supported in the visitor

[kfriedberger]

lgtm