Integer functions

Adds support (freezing, typing, printing) for the following integer functions from sympyextensions.integer_functions:

• bit_shift_left
• bit_shift_right
• bitwise_and
• bitwise_or
• bitwise_xor
• int_div
• int_power_of_2
• modulo_floor

The following are left for future work because they can be conveniently implemented with an if expression (the ternary if operator ?: in C) which is currently not available.

• modulo_ceil
• div_ceil

div_floor is also not implemented because to me the semantics of this function are unclear. Is it supposed to compute

\lfloor \frac ab \rfloor

(as the name suggests) or the C-like round-to-zero integer division (as the documentation suggests)?

The typifier tries to infer the integer type of the arguments/result and always raises if the type is non-integer. Note that this can lead to cases in which an exception is raised because the inference fails. For example int_div(2, 3) where both arguments are untyped constants. This could be improved in the typifier, e.g. by adding type constraints to deferred constants (also left for future work).