Use "growing" `GArray` to store string field's payload

Use "growing" `GArray` to store string field's payload

Minimize the number of required operations for functions such as
bt_field_string_set_value(), bt_field_string_append(),
bt_field_string_append_len(), and bt_field_string_clear().

This patch makes the string field object use the same principle that the
sequence field uses, but with a `GArray` of characters instead of a
`GPtrArray` of fields.

Preliminary benchmarks show that this approach is faster than using a
`GString` object.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>