Move more internal components into util folder

[st0012]

Assuming the top-level include/rbs holds public components (e.g. parser.h), and include/rbs/util holds internal components, more stuff should be moved under util.

This follows the same convention prism uses.

Before

include
├── rbs
│   ├── ast.h
│   ├── defines.h
│   ├── encoding.h
│   ├── lexer.h
│   ├── parser.h
│   ├── rbs_buffer.h
│   ├── rbs_encoding.h
│   ├── rbs_location.h
│   ├── rbs_location_internals.h
│   ├── rbs_string.h
│   ├── rbs_strncasecmp.h
│   ├── rbs_unescape.h
│   └── util
│       ├── rbs_allocator.h
│       ├── rbs_assert.h
│       └── rbs_constant_pool.h
└── rbs.h

After

include
├── rbs
│   ├── ast.h
│   ├── defines.h
│   ├── lexer.h
│   ├── parser.h
│   ├── rbs_location.h
│   ├── rbs_string.h
│   └── util
│       ├── rbs_allocator.h
│       ├── rbs_assert.h
│       ├── rbs_buffer.h
│       ├── rbs_constant_pool.h
│       ├── rbs_encoding.h
│       └── rbs_unescape.h
└── rbs.h

[amomchilov]

Aside, but this is a weird comment. There is no flexible array member here (that would be rbs_loc_entry entries[]), and there's no allocation there, either.

[st0012]

FWIW this is the PR introduced the usage: ruby#1786