add clear() function that replaces Main

a link to the previous Main called LastMain is provided.
`using LastMain.Package` works to restore access to loaded libraries.

fixes #1195, fixes #2385

base/exports.jl

@@ -1058,6 +1058,7 @@ export
     versioninfo,
     which,
     whos,
+    clear,
 
 # loading source files
     evalfile,

base/interactiveutil.jl

@@ -330,3 +330,16 @@ function download(url::String)
     filename = tempname()
     download(url, filename)
 end
+
+function clear()
+    last = Core.Main
+    b = last.Base
+    ccall(:jl_new_main_module, Any, ())
+    m = Core.Main
+    ccall(:jl_add_standard_imports, Void, (Any,), m)
+    eval(m,
+         Expr(:toplevel,
+              :(const Base = $(Expr(:quote, b))),
+              :(const LastMain = $(Expr(:quote, last)))))
+    m
+end

base/reflection.jl

@@ -3,8 +3,26 @@ module_name(m::Module) = ccall(:jl_module_name, Any, (Any,), m)::Symbol
 module_parent(m::Module) = ccall(:jl_module_parent, Any, (Any,), m)::Module
 current_module() = ccall(:jl_get_current_module, Any, ())::Module
 
-fullname(m::Module) = m===Main ? () : tuple(fullname(module_parent(m))...,
-                                            module_name(m))
+function fullname(m::Module)
+    if m === Main
+        ()
+    elseif module_parent(m) === m
+        # not Main, but is its own parent, means a prior Main module
+        n = ()
+        this = Main
+        while this !== m
+            if isdefined(this, :LastMain)
+                n = tuple(n..., :LastMain)
+                this = this.LastMain
+            else
+                error("no reference to module ", module_name(m))
+            end
+        end
+        return n
+    else
+        tuple(fullname(module_parent(m))..., module_name(m))
+    end
+end
 
 names(m::Module, all::Bool, imported::Bool) = ccall(:jl_module_names, Array{Symbol,1}, (Any,Int32,Int32), m, all, imported)
 names(m::Module, all::Bool) = names(m, all, false)

src/julia.h

@@ -808,7 +808,7 @@ DLLEXPORT void jl_module_import(jl_module_t *to, jl_module_t *from, jl_sym_t *s)
 DLLEXPORT void jl_module_importall(jl_module_t *to, jl_module_t *from);
 DLLEXPORT void jl_module_export(jl_module_t *from, jl_sym_t *s);
 DLLEXPORT jl_module_t *jl_new_main_module(void);
-void jl_add_standard_imports(jl_module_t *m);
+DLLEXPORT void jl_add_standard_imports(jl_module_t *m);
 STATIC_INLINE jl_function_t *jl_get_function(jl_module_t *m, const char *name)
 {
     return  (jl_function_t*) jl_get_global(m, jl_symbol(name));