In exponent(), 'base *= base;' should not run
when base is 65536, since it overflows an int32_t.
This also optimizes exponent() based on
gcc and clang -O3 test cases in godbolt.org.
Document the existing `ROUND`, `CEIL`, and `FLOOR` functions
Also update the trig function docs for searchability
Implement `POW` and `LOG`
Addresses part of #675
Implement ** for integer exponents
** has higher precedence than -, like Python, so -3**4 == -(3**4) == 81
