support inline across files

[glathoud]

function declarations only + function name must be unique else error (we only parse and change the code, we do not execute it at this point).

About closures:

• forbid inlining of a function from another file when the function body uses some external variables (closure).
• permit closure when inlining a function within the same file as long as there is no ambiguity on the external variables. Else forbid (error).

[glathoud]

todo: closure check in the various cases.

[glathoud]

Actually we would want NOT to forbid global access to "externals" (e.g. to some other tools) but only to forbid access to a locally bound variable.