Do not require surjectivity to be implemented for composite injectivity

[saraedum]

Currently, this fails even though all morphisms are known to be injective.

sage: QQ.hom(QQbar)*ZZ.hom(QQ)

Composite map:
  From: Integer Ring
  To:   Algebraic Field
  Defn:   Ring Coercion morphism:
          From: Integer Ring
          To:   Rational Field
        then
          Ring Coercion morphism:
          From: Rational Field
          To:   Algebraic Field
sage: _.is_injective()

Component: commutative algebra

Keywords: sd86.5

Author: Julian Rüth

Branch/Commit: b21c370

Reviewer: David Roe

Issue created by migration from https://trac.sagemath.org/ticket/23205

[saraedum]

Branch: u/saraedum/do_not_require_surjectivity_to_be_implemented_for_composite_injectivity

[saraedum]

New commits:

b21c370

Do not require is_surjective/is_injective to be implemented on as many morphisms to decide injectivity/surjectivity of composite maps

[saraedum]

Commit: b21c370

[roed314]

Reviewer: David Roe

[roed314]

comment:3

All tests pass; looks good.

[vbraun]

Changed branch from u/saraedum/do_not_require_surjectivity_to_be_implemented_for_composite_injectivity to b21c370