Basic expression evaluation using go/parser.

Implemented operations:

• +x: nop
• -x: arithmetic negate
• ^x: bitwise not
• x+y: sum
• x-y: difference
• x*y: product
• x/y: quotient
• x%y: remainder of division
• x^y: bitwise exclusive or
• x&y: bitwise and
• x|y: bitwise inclusive or
• x&^y: bit clear
• x<<y: bitwise shift left
• x>>y: bitwise (arithmetic) shift right
• abs(x): absolute value
• binomial(x, y): binomial coefficient (x choose y)
• lb(x): position of highest set bit
• pow(x, y[, m]): exponentiation base x of y, optionally mod m
• gcd(x, y): greatest common denominator
• modinv(x, y): multiplicative inverse of x in the group Z/yZ where y is assumed prime
• factorial(x=1, y): y!/x!
• rand(x=0, y): random number in [x, y)