use Kernel#__send__ insteard of Kernel#send
Implement Struct#to_h .
Merge pull request #2162 from ksss/__send__

use Kernel#__send__ insteard of Kernel#send
Merge pull request #2163 from take-cheeze/struct_to_h

Implement Struct#to_h .
Implement header search for gcc and clang.
Use header searcher to find readline and linenoise.