style(types/formula/*): Tabs to spaces
fix(types/formula/term.cpp): Var type declaration

Returns void since returned term is useless. Change to set since
returns void.
fix(types/formula/term.cpp): Persistent var value

Should be able to change variable type after declaring its value.
feat(src/types/*): Add utility functions
chore(*): Change *_type to *_op for consistency
chore(*): Change *_type to *_op for consistency