refactor FileType so compressed and archived file handling is transparent

[jtmoon79]

Summary

Currently enum FileType notion of compressed and archived files presumes those to be plain-text ad-hoc log files ("syslog" files).

Refactor enum FileType to have variants to signify the other file types, FixedStruct, Journal, etc., may also be a compressed or archived.

Current behavior

Currently, a compressed file, log.gz, is given FileType::Gz. It is presumed to be an ad-hoc text log ("syslog" file). This presumption means a compressed fixed struct file, wtmp.gz, is presumed to be a compressed text log file and parsing fails.

pub enum FileType {
    Unset,
    File,
    Gz,
    Tar,
    TarGz,
    Xz,
    FixedStruct { type_: FixedStructFileType },
    Evtx,
    Journal,
    Unparsable,
    Unknown,
}

Suggested behavior

enum FileType and BlockReader should be refactored to more transparently handle files that are compressed or archived. Compression and archival should be generic sub-variants for primary FileType files.

pub enum FileType {
    Unset,
    Text { archival_type: FileArchivalType },
    FixedStruct { archival_type: FileArchivalType, type_: FixedStructFileType },
    Evtx { archival_type: FileArchivalType },
    Journal { archival_type: FileArchivalType },
    Unparsable,
    Unknown,
}
pub enum FileArchivalType {
    Normal,
    Gz,
    Xz,
    Tar,
}

Issue #283 adds file-searching "seeking modes" (sequential seeking or random seeking). That "seeking mode" determination should be based on these proposed enum values.

Other Considerations

FileType::Journal

A FileType::Journal that is compressed or archived (archival_type != Normal) would need to be handled by extracting to a named temporary file (Issue #284).

Multiple levels of archiving or compression

This proposed design presumes "one level" of archiving or compression, e.g. can process file log.tar but cannot process file log.tar.xz. Chained Block Readers (Issue #14) implementation would follow this implementation. Chained Block Readers would require a stack of FileArchivalType held or referenced by the FileType value, e.g. can process file log.tar.xz.

Encoding

While working on this issue, can also cover Issue #16; add to FileType::Text a sub-variant text_encoding.

pub enum TextEncodingType {
    Utf8Ascii,
    Utf16,
    Utf32,
}