conversion SR to asymptotic ring

[cheuberg]

It should be possible to convert a symbolic expression to an asymptotic expansion:

sage: A.<x1> = AsymptoticRing('x^QQ', QQ)
sage: A(x^2+x+1)
sage: A((x + 1)/(x - 1))
Traceback (most recent call last):
...
ValueError: (x + 1)/(x - 1) is not in Exact Term Monoid x^QQ with coefficients in Rational Field.
> *previous* ValueError: Factor x + 1 of (x + 1)/(x - 1) is neither a coefficient (in Rational Field) nor growth (in Growth Group x^QQ).
sage: ((x+1)/(x-1)).subs(x=x1)
Traceback (most recent call last):
...
TypeError: no canonical coercion from Asymptotic Ring <x^QQ>
over Rational Field to Symbolic Ring

Substitution is probably harder than conversion. Related to #19429.

CC: @dkrenn @behackl

Component: asymptotic expansions

Issue created by migration from https://trac.sagemath.org/ticket/19947