Remove all Context.Global* methods

[meatballhat]

and change the behavior of the non-Global variants to always search up the
context lineage.

Closes #385

[jszwedko]

Feels like you could shrink this up by doing something like:

for cur := c; cur != nil; cur = cur.parentContext {
  lineage = append(lineage, cur)
}

[jszwedko]

👏 I'm super happy to see this.

Do you think you could add a couple of tests testing:

• context.Lineage
• That the context accessor functions properly traverse up (or maybe just test lookupFlagSet since that is where most of the logic is)
• That IsSet traverses up

❤️

[jszwedko]

👏 This reduced signature feels much better.

[jszwedko]

Beautiful.