fix coefficients for symbolic series

[rwst]

There is no SR.function that gets the coefficients of SR.series; rather, coeffs() etc give confusing results; truncate() must be prepended manually.

sage: x=var('x')
sage: s=(1/(1-x)).series(x,6)
sage: s.coeffs()
[[x^5 + x^4 + x^3 + x^2 + x + Order(x^6) + 1, 0]]
sage: s.coefficient(4)
0

Working around with s.truncate() is only a fig leaf.

Extracted from http://ask.sagemath.org/question/24968/coefficients-in-polynomial-ring-over-symbolic-ring/

Depends on #17438

CC: @kcrisman

Component: symbolics

Author: Ralf Stephan

Branch/Commit: 0b9914f

Reviewer: Bruno Grenet

Issue created by migration from https://trac.sagemath.org/ticket/17399

[rwst]

Description changed:

--- 
+++ 
@@ -9,3 +9,5 @@
 0
 ```
 Working around with `s.truncate()` is only a fig leaf.
+
+Extracted from http://ask.sagemath.org/question/24968/coefficients-in-polynomial-ring-over-symbolic-ring/

[rwst]

Branch: u/rws/fix_coefficients_for_symbolic_series

[rwst]

New commits:

32ae67c

17399: do not let maxima handle ex.series coefficients

[rwst]

Commit: 32ae67c

[rwst]

Author: Ralf Stephan

[rwst]

Dependencies: #17438

[sagetrac-git]

Changed commit from 32ae67c to 6cd5286

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

99820cf	17399: roll back previous commit to allow merge of 17428
d7da52c	17438: implement coeff list
9452fa9	17438: deprecate ex.coeff/coeffs()
0fec129	17438: implement ex.list()
168b659	Merge branch 'u/rws/coefficients_of_symbolic_expressions_revamp' of trac.sagemath.org:sage into t/17399/fix_coefficients_for_symbolic_series
6cd5286	17399: handle series in ex.coefficients()