check for subs of AsymptoticExpansion

[dkrenn]

sage: A.<n> = AsymptoticRing('n^ZZ', QQ, default_prec=3)
sage: n
n
sage: _.subs(n=1/(2-1/n))
1/2 + 1/4*n^(-1) + 1/8*n^(-2) + O(n^(-3))
sage: _.subs(n=1/(2-1/n))
O(1)

is wrong (the two substitutions are equal to the identity).

Similarly,

sage: n.subs(n=1/(2-1/n))
1/2 + 1/4*n^(-1) + 1/8*n^(-2) + O(n^(-3))
sage: _.subs(n=A(1/2))
O(1)

Aim: add a check 1/f is o(1) to .subs(n=f).

CC: @cheuberg @behackl

Component: asymptotic expansions

Issue created by migration from https://trac.sagemath.org/ticket/22414