Changes

This is the next step for simplifying temp reading in contexts.
- add #indexFromIR
- use it in what was readFromContext:scope:, rename it to
#readFromContext: (as it does need the scope)
- rename #searchFromContext:scope: to be #readFromContext:scope:
- for arguments: make clear that writing is not possible
Much better... more steps will be done after this (the goal is to remove
the #scope: parameter and make it even simpler)

- simplify #lookupAndReadVar: inContext: to call itself for recursion,
not the helper method - this allows us to remove  #outerScopeLookupVar:
inContext: - first step to fix copying temp writing: update the orginal
var, too