Add length property to cartesianProduct for finite forward ranges

[manavgupta26]

Fixes issue 9871

[dlang-bot]

Thanks for your pull request and interest in making D better, @manavgupta26! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

⚠️⚠️⚠️ Warnings ⚠️⚠️⚠️

• In preparation for migrating from Bugzilla to GitHub Issues, the issue reference syntax has changed. Please add the word "Bugzilla" to issue references. For example, Fix Bugzilla Issue 12345 or Fix Bugzilla 12345.(Reminder: the edit needs to be done in the Git commit message, not the GitHub pull request.)

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#10961"

[pbackus]

Needs a unit test.

[pbackus]

Suggested change

	import std.algorithm : allSatisfy;
	import std.meta : allSatisfy;

This should fix the style check failure.

[pbackus]

std/algorithm/setops.d(425): Error: mutable method `std.algorithm.setops.__unittest_L575_C7.MapResult!(__lambda_L578_C28, int[]).MapResult.ImplementLength!(_input).length` is not callable using a `const` object
                   result *= r.length;
                             ^

The problem is that cartesianProduct's length method is const, but MapResult's length method is not.