This prevent the assumption from being dragged into lemmas that do not even need it

The singleton maps notation is now also more consistent with the
insert <[_ := _]> _ notation for maps.

Also, put stuff in a section.

* These type classes bundle an identifier into the global CMRA with a proof
  that the identifier points to the correct CMRA. Bundling allows us to get
  rid of many arguments everywhere.

* I have setup the type classes so that we no longer have to keep track of the
  global CMRA identifiers. These are implicit and resolved automatically.

* For heap I am also bundling the name of the heap RA instance. There always
  should be at most one heap instance so this does not introduce ambiguities.

* We now have a "maps to" notation!

this uncovered that our story with respect to disjointness of namespaces is still lacking

* Clearly separate the file algebra/sts in three parts:
  1.) The definition of an STS, step relations, and closure stuff
  2.) The construction as a disjoint RA (this module should never be used)
  3.) The construction as a CMRA with many derived properties
* Turn stsT into a canonical structure so that we can make more of its arguments
  implicit.
* Rename the underlying step relation of STSs to prim_step (similar naming as
  for languages, but here in a module to avoid ambiguity)
* Refactor program_logic/sts by moving general properties of the STS CMRA to
  algebra/sts.v
* Make naming and use of modules in program_logic/sts more consistent with
  program_logic/auth and program_logic/saved_prop
* Prove setoid properties of all definitions in program_logic/sts

These are no longer needed, since fill is no longer a type class.

Remarks:
* eauto needs more fuel to automatically solve the side-conditions.
* ssreflect rewrite works if we do a set (φ ..) first. No idea why.

...unfortunately, that proof actually got longer because some automation no longer works