Add struct `Convert` for converting meta data

[bvssvni]

Currently there are a bunch of methods taking &[Range<MetaData>], offset: usize. This could be put in a struct Convert to encapsulate the state. Could use same pattern as in the read_token library.

pub struct Convert<'a> {
    data: &'a [Range<MetaData>],
    offset: usize,
}

Move functions to methods on Convert:

• [x] update
• [x] start_node
• [x] end_node
• [x] ignore
• [x] meta_string
• [x] meta_f64
• [x] meta_bool

Use Convert internally in convert:

• [x] read_string
• [x] read_sequence
• [x] read_set
• [x] read_until_any_or_whitespace
• [x] read_until_any
• [x] read_tag
• [x] read_whitespace
• [x] read_text
• [x] read_number
• [x] read_reference
• [x] read_select
• [x] read_optional
• [x] read_separated_by
• [x] read_lines
• [x] read_repeat
• [x] read_rule
• [x] read_node