This commit completely redesigns zone file parsing. The primary change is to
convert the scanner into a trait in order to allow multiple zone parser
implementations for different sources and purposes. A number of changes had
to be made in order to make this possible.
The commit also contains an initial implementation of a scanner that modifies
data in-place and can thus returned parsed data with only a minimal amount
of additional allocations. While working, this scanner is more a
proof-of-concept at this point to ensure that the API design is sound.
This commit is based on ideas and code proposed by @not-my-profile in #106.
This commit increases the minimal supported Rust version to 1.59.0.
