[assignment_to_subst] rename subst arguments

This avoids confusion with the names already present in the kernel.
Earlier:
```
t_unit = lp.make_kernel(
    "{[i]: 0<=i<10}",
    """
    <> tmp[i] = 2*i
    out[i] = 2*tmp[i]
    """)
t_unit = lp.assignment_to_subst(t_unit, "tmp")
```
would yield:
```
---------------------------------------
DOMAINS:
{ [i] : 0 <= i <= 9 }
---------------------------------------
INAME TAGS:
i: None
---------------------------------------
SUBSTITUTION RULES:
tmp_subst(i) := 2*i
---------------------------------------
INSTRUCTIONS:
for i
  out[i] = 2*tmp_subst(i)  {id=insn_0}
end i
---------------------------------------
```

Notice how the substitution argument is 'i' and the kernel contains 'i'
as a domain dim.