Make String:null and ByteVector::null const.
Merge pull request #111 from TsudaKageyu/null

Make String:null and ByteVector::null const.
Avoid resizing buffer frequently in String::data()
Merge pull request #112 from TsudaKageyu/string-data

Avoid resizing a buffer frequently in String::data()
Fix uses of ulong where 32-bit integers are expected
Merge pull request #113 from TsudaKageyu/fix-ulong

Fix uses of ulong where 32-bit integers are expected