fix: allow yaegi command to interpret itself

Since the introduction of restricted stdlib and syscall symbols, the
capability of yaegi to interpret itself was broken.
The use of unrestricted symbols is now also controlled by environment
variables, to allow propagation accross nested interpreters.
The interpreter Panic symbol was not wrapped, this is fixed now.
the import path resolution was failing if the working directory was
outside of GOPATH.
The documentation and readme have been ajusted.

Fixes #890.

interp/run.go

@@ -2037,7 +2037,7 @@ func doCompositeBinStruct(n *node, hasType bool) {
 			}
 		} else {
 			fieldIndex[i] = []int{i}
-			if c.typ.cat == funcT {
+			if c.typ.cat == funcT && len(c.child) > 1 {
 				convertLiteralValue(c.child[1], typ.Field(i).Type)
 				values[i] = genFunctionWrapper(c.child[1])
 			} else {