Because a structure that is an element of `presym_table` has padding, split
it into individual arrays for name and length.

#### Result (64-bit CPU with full-core gembox)

|        |   mruby    | libmruby.a |
|--------|------------|------------|
| Before | 1,087,444B | 1,476,872B |
| After  | 1,079,340B | 1,469,784B |