Remove conn requirement from generated xproto

The conn dependency was unneeded.
Regenerate xproto
Make the timeout for poll-event in milliseconds

Make the unit of timeout for poll-event to be always in milliseconds.
Make to-frame/to-value return raw frame/value

Make to-frame/to-value return raw frame/value, i.e. content of request
without opcode, length, or extra padding. The generation of complete
frames/values is now the responsibility of send, which now resides in
conn-internal instead of gen-common.

This change is made as to decrease the amount of code in xmlgen and to
simplify the implementation of extensions in the future, which contains
complications such as major/minor opcodes.
Regenerate xproto
Renaming gen-common to common

gen-common was originally created to contain everything that a generated
file would need, such as by xproto. The name became a misnomer when it
became apparent that other (hand-written) files also need the functions
contained within. This renaming makes the name match its actual usage.
Make sequence numbers in connection a lazy-seq

The sequence numbers in connections are now a lazy-seq of all the
permissible sequence numbers. The sequence number for the next request
is simply the head of the lazy-seq.
Represent XIDs in connections as lazy-seq

Represent XIDs in connections as lazy-seq, with the head as the next
valid XID. This is done to simplify gen-xid, and to make adding more
XIDs more convenient (e.g. through XC-MISC).
Change connection object from atom of map to map

The connection object itself does not need to be modified, since the
only parts of the map that need to be modified are atom/ref (seq-nums
and xids).