metadata: always add/store/retrieve size for links and normal files

metadata: always add/store/retrieve size for links and normal files

This simplifies cases where we need to transmit Metadata
objects (i.e. bup-get's repo.resolve()), and it means that for trees
created using this new v3 format, retrieving the sizes of chunked
files should be notably less expensive, since they'll be directly
available in the directory's .bupm file.

Without that, we have to seek around in the chunked tree to find the
last byte (cf. vfs._normal_or_chunked_file_size).

Only store the sizes for symlinks and regular files (which might be
chunked) until it's clear that other st_sizes are useful.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>