struct iseq_compile_data_storage: 16 bytes (from 32) overhead

This reduces the iseq_compile_data_storage header from 32 to 16
bytes on 64-bit systems.

pos and size fields cannot exceed 32-bit sizes due to stack size
limits.  Using a flexible array for the buffer also saves us 8
bytes of pointer overhead.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46958 b2dd03c8-39d4-4d8f-98ff-823fe69b080e