Uniform bead references

[e3krisztian]

bead-reference is one of:

• name of an existing file
• [box:]name[@absolute-version]
• @relative-version (relative references to existing input)

Where absolute-version is one of:

• unique part of timestamp-prefix
• unique part of contentid after # (# is to disambiguater from timestamp prefix)

Relative-version is absolute-version and the following

• next / +
• prev / previous / -

bead-reference examples:

• ~/beads/xyt.zip (actually ~ should be resolved by the shell)
• default, most recent version available
  • name
  • box:name
• unique timestamp prefix
  • name@202009
  • name@20200920T1113
• unique contentid part
  • name@#202009
• relative versions (useful for update)
  • @next
  • @prev
  • @previous
  • @+
  • @--