Simplify sharing contexts at top level

[fornellas]

If we have a shared context defined in one file:

def some_shared_context(context):
  @context.before
  def before(self):
    pass

and we want to use it at different files, we need to do this:

from wherever import some_shared_context

@context
def first(context):
  context.shared_context(some_shared_context)
  context.merge_context("some shared context")

and on the other file:

from wherever import some_shared_context

@context
def second(context):
  context.shared_context(some_shared_context)
  context.merge_context("some shared context")

We can somewhat easily modify the merge_context function to accept the function directly, simplifying things:

from wherever import some_shared_context

@context
def first(context):
  context.merge_context(some_shared_context)

and on the other file

from wherever import some_shared_context

@context
def second(context):
  context.merge_context(some_shared_context)

[xush6528]

I think it's equivalent to make all contexts sharable by default (at least conceptually).

[fornellas]

Shared contexts should not be executed, unless they're put into another "real" context. Whether or not "real" contexts can / should be shareable, it seems to me that is a different issue than the one described here.