feat(analysis/normed_space/operator_norm): extension of a uniform continuous function (#1649)
feat(analysis/normed_space/operator_norm): extension of a uniform continuous function (#1649)

* Extension of a uniform continuous function

* Use characteristic properties of an extended function, instead of the explicit construction

* Add documentation on similar results in the library

* Update src/topology/algebra/uniform_extension.lean

Co-Authored-By: sgouezel <sebastien.gouezel@univ-rennes1.fr>

* Travis failed for no reason

* Update uniform_extension.lean

* eliminate `uniform_extension.lean`

* Update operator_norm.lean

* Update operator_norm.lean

* Remove `M`

* Fix docstring; extend_zero should be a simp lemma

leanprover-community/mathlib,leanprover-community/mathlib