Fix type argument inference of session.get(key) and session.set(key, value)

[zetaraku]

The current generic method session.get(key) accidentally allows the return value to be inferred into any type.

The session.set(key, value) also allows value of any type to be passed in.

Note: This fix removes the second generic param V, so it will break an undocumented usage for TypeScript users specifying both two generic type parameters. (This should be rare.)

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm