Move importInfo to Context store
Move importInfo to Context store

Stats show that importInfo is updated about 10x less that Store.
E.g. compiling Dotty:

Total context creations: 5.6M
Store creations:          68K
Import info creations:   6.8K

So this means we save 22.4M be removing importInfo as a field of contexts
at a price of less than 600K for the added store creations.

# Conflicts:
#	compiler/src/dotty/tools/dotc/core/Contexts.scala

sjrd/dotty,dotty-staging/dotty,dotty-staging/dotty,lampepfl/dotty,dotty-staging/dotty,dotty-staging/dotty,lampepfl/dotty,lampepfl/dotty,sjrd/dotty,sjrd/dotty,lampepfl/dotty,sjrd/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty