GHCi fixes:

  - expressions are now compiled in a pseudo-module "$Interactive",
    which avoids some problems with storage of demand-loaded declarations.

  - compilation manager now detects when it needs to read the interace
    for a module, even if it is already compiled.  GHCi never demand-loads
    interfaces now.

  - (from Simon PJ) fix a problem with the recompilation checker, which
    meant that modules were sometimes not recompiled when they should
    have been.

  - ByteCodeGen/Link: move linker related stuff into ByteCodeLink.