Make HeScheme context flexible

[fboemer]

Make HeContext protocol, which different HE schemes can implement.

[karulont]

Introduction of HeContext is good.
But I see no big value in removing HeScheme from Context.
Context<Scheme: HeScheme> is in my opinion better than Context<Scalar> and then passing in Scheme separately.

[fboemer]

Introduction of HeContext is good. But I see no big value in removing HeScheme from Context. Context<Scheme: HeScheme> is in my opinion better than Context<Scalar> and then passing in Scheme separately.

SGTM; faa9a32 has the delta.

[karulont]

Removing Scheme from Context required a lot of extra type annotations. Now that we put the scheme back into context. Those type annotations are not needed, and I would rather like to keep the change small and revert unnecessary changes.

[fboemer]

This works:

    public static func multiplyInverseTest<Context: HeContext>(context: Context) async throws where Context.Scheme.Context == Context {

Note, the more natural

    public static func multiplyInverseTest<Scheme: HeScheme>(context: Scheme.Context) async throws {

fails due to Generic parameter Scheme is not used in function signature.

We previously had

    public static func multiplyInverseTest<Scheme: HeScheme>(context: Context<Scheme>) async throws {

but that constrains use of the function to schemes for which Scheme.Context == Context<Scheme>