Use same slug in different taxonomies

[scribu]

[13087] was made in order to prevent sharing terms across taxonomies.

I'm not proposing sharing terms, only sharing slugs. In other words, to be able to have this:

Taxonomy   Name     Slug

post_tag   foo-bar  foobar
category   Foobar   foobar

Related:

Related: #12269, #11838

I now see that unique terms are enforced at the schema level (in wp_terms):

From IRC:

	<rboren>	If it involves changing the schema it will probably never, ever happen. :-)
	<scribu>	it doesn't necessarily involve changing the schema
	<scribu>	just some changes to wp_unique_term_slug() and probably to wp_insert_term()
	<scribu>	btw, why are you guys so adamant about keeping the schema?
	<rboren>	Because it was hell to change to the current one for taxonomy.
	<rboren>	MU sites were painful to move over.
	<rboren>	And the current schema is the way it is mainly for MU sites.
	<rboren>	Canonical slugs across taxonomies was a requirement, at the time.
	<rboren>	Which led to some less than nice slug juggling for hierarchical taxonomies.
	<rboren>	That's the main part people don't like, the slug mangling for subcats.
	<rboren>	Those who were around for the great taxonomy war still bear scars. :-)

I now see that this constraint would have to be removed at the schema level, from wp_terms:

UNIQUE KEY slug (slug)

Related: #13119

#6211