Enable support for watchosX64

[goncalossilva]

Supersedes #93, with the changes:

• Enables watchosX64 in both modules, not just core.
• Rebased from master, fixing inconsistencies with the stdlib (1.4.30 and 1.5.0 are source-incompatible, hence the CI failure in Support x86_64-based watchOS simulator target #93).
• The above enables switching to kotlinx.serialization 1.2.1 (watchosX64 is only supported on 1.2.0 and up).
• Simplified setup, with kotlinx.serialization being declared only once.
• Test suite should be green, but I can't trigger TeamCity. 😄

[goncalossilva]

Is there any chance this might be reviewed soon?

We have an internal milestone by the end of the month that needs this... We can package and publish this internally for demonstration purposes, but if we have a chance to work with the official build, we'd take it. Thanks!

[dkhalanskyjb]

Simplified setup, with kotlinx.serialization being declared only once.

The complex setup that we have is intentional. Its purpose is so that the JVM version doesn't introduce a dependency on the serialization library. So, we have to make it work with a compileOnly dependency in the common code.

We will need to make a new release in order to fix #118, and watchosX64 support would likely fit there nicely, so, if everything goes well, we'll have a release by the end of the week that introduces it.

[goncalossilva]

Simplified setup, with kotlinx.serialization being declared only once.

The complex setup that we have is intentional. Its purpose is so that the JVM version doesn't introduce a dependency on the serialization library. So, we have to make it work with a compileOnly dependency in the common code.

That should be an easy change, but it's probably not worth it if you're planning to fix it as part of #118?

[goncalossilva]

Changes in #120.