implement compiler directives

[Kacarott]

[JohanWiltink]

in no particular order:
#import filename.lc - import external code ( may need facility for specific imports renaming or namespacing )
#export binding-name - export a subset of defined bindings
#no-export binding-name - export a subset of defined bindings, inverted logic
#constant binding-name - protect a binding from being overwritten in the current scope
#define binding-name JS-code - define a magic binding