fix!: immutable headers guards for incoming response and downstream request

[guybedford]

This implements immutable headers guards for the fetch event request, and any fetch() responses.

With this change, trying to set a header on one of these will throw a TypeError as is correct per the fetch and service worker specs.

Resolves #992.

[guybedford]

This should also reintroduce the "fetch/api/response/response-headers-guard.any.js" WPT test.

[guybedford]

This can be done as a minor since there is no functional change, so long as it rebased in a way that respects the Candidate Response phase of the Response permitting mutations (i.e. the definition of is_upstream needs to be tweaked, and the guard needs to change during the lifecycle).

[cfallin]

Could someone remove me as the requested reviewer? (@sunfishcode?) I'm cleaning up old review requests and still have this one on my queue but no permissions to remove myself or close it anymore.

[cfallin]

@cceckman-at-fastly @dgohman-fastly as two recent committers, it looks like you have rights to do so -- could you please remove me as a reviewer on this old PR?

[dgohman-fastly]

@cfallin I've now removed you as a reviewer.