static compile round 2

This is largely complete. I'll probably try to merge it piecemeal, but I thought people might like to see the status of this.

Big TODO items remaining:
- [x] respect the jl_set_imaging_mode flag in some essential places (currently only sys0.ji gets the benefits of this)
- [x] clean-up some platform-specific code (currently targets Mac)
- [x] precompile a much greater set of functions
- [x] lookup all external / ccall functions at runtime
- [x] add support for windows
- [x] improve efficiency of ccall function lookup
- [x] investigate cause of repl crash when executing the first input line
- [x] enough comments to make Jeff happy
- [x] jl_load_and_lookup(libname, funcname, libptrgv)
- [x] reduce size of sysimg file such that this pull request doesn't affect the size of the file
- [x] misc cleanup requested by Jeff