[ruby/yarp] refactor: extract `peek_addr()`

In many places in the code we use the idiom:

    x < parser->end && *x == 'y'

which is essentially an extension of an existing pattern:

- `peek()` looks at `parser->current.end`
- `peek_at()` looks at `(parser->current.end + offset)`

This commit introduces a new inline function, `peek_addr`, which
accepts a pointer and encapsulates the address value check and
conditional dereferencing. The result is more readable code, and more
ubiquitous safety checks on pointer values, allowing us to rewrite the
above as:

    peek_addr(parser, x) == 'y'

Also:

- change the type of `peek_at()`'s offset argument from `size_t` to
  `ptrdiff_t` so that it can accept negative offsets.
- use `current_token_starts_line` in one place where the equivalent
  code is inline.
- use `peek` or `peek_at` to replace inline code in a few places

These changes simplify the code and make it easier to visually spot
patterns, particularly around line-endings (which will be a subject of
a future pull request).

https://github.com/ruby/yarp/commit/4c608d53ea

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