[Nitpicks] Fix doc comments and code styles
[Refactoring] Delaying throwing Exception makes control flow analysis simple
[appender] Reuse ensureAddable for reserve implementation

AFAICS, they have mostly same code.
[appender] Accepting qualified buffer is not safe behavior

To reduce existing code breaking, mark such construction as @system.
[appender] Determine @trusted operations
Temporary stop using std.conv.to, text, and std.string.format at several critical points
[parse] Make @safe pure
[formatValue] (for integral)