Improve speed of types

[ehmicky]

This PR improves the speed of the types. On my machine, they are now ~4 times faster.

Before:

$ time tsc index.d.ts
real	0m5.064s
user	0m7.940s
sys	0m0.339s

After:

$ time tsc index.d.ts
real	0m1.284s
user	0m3.242s
sys	0m0.133s

Also, our type checking task is now roughly twice faster.

What this PR does is removing a few extends constraints. Apparently, some of them were making the types much slower, but were more for safety/documentation purpose, i.e. could be removed without impacting the types. All of those types are internal.