improve zk-DSL docs + small syntax changes

[TomWambsgans]

Two small breaking changes in the zkDSL syntax:

function arguments can no longer be 'Mut'

previously:

def my_func(a, b: Mut, c: Const):
    ...

now:

def my_func(a, b_temp, c: Const):
    b: Mut = b_temp
    ...

Rename 'Imu' to 'Imm'

def my_func(a, b):
    x: Imu
    if a == 0:
         x = 3
    else:
         x = 4
    ...

now:

def my_func(a, b):
    x: Imm
    if a == 0:
         x = 3
    else:
         x = 4
    ...