Move 15 loose scripts from contrib/ root to contrib/scripts/

Consolidate all standalone utility scripts into contrib/scripts/:
- Perl: avg_blocktime.pl, block_time.pl, gen-zaddrs.pl, sda_checkpoints.pl,
  sdl_checkpoints.pl, hush_block_subsidy_per_halving, hush_halvings,
  hush_scanner, hush_supply, hush_supply_old
- Shell: fresh_clone_compile_and_run.sh, tidy_datadir.sh, dragonx_scanner
- Python: convert_address.py
- BAT: hush-uri.bat

Update path references in contrib/README.md, doc/OLD_WALLETS.md,
doc/relnotes/README.md, and sdl_checkpoints.pl.

doc/relnotes/README.md

@@ -231,8 +231,8 @@ since it contains various bugfixes and improvements that will benefit busy walle
   * sendmany RPC will now reject transactions that send to taddrs immediately, instead of them being rejected in mempool
   * Preliminary support for FreeBSD has been added to the Hush build system
   * New contrib scripts:
-    * `contrib/gen-zaddrs.pl` - Generate zaddrs in bulk, defaults to 50
-    * `contrib/sdl_checkpoints.pl` - Generate SDL checkpoints using `getblockmerkletree`
+    * `contrib/scripts/gen-zaddrs.pl` - Generate zaddrs in bulk, defaults to 50
+    * `contrib/scripts/sdl_checkpoints.pl` - Generate SDL checkpoints using `getblockmerkletree`
   * ZeroMQ support has been removed from Hush
 
 # Hush 3.9.1 "Luciferous Locust"