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.

contrib/README.md

@@ -14,7 +14,7 @@ Estimate when a Hush block will happen.
 
 Example:
 
-    ./contrib/block_time.pl  123456 # Print out datetime of when block height 123456 happens
+    ./contrib/scripts/block_time.pl  123456 # Print out datetime of when block height 123456 happens
 
 ## gen-zaddrs.pl
 
@@ -22,8 +22,8 @@ Generate zaddrs in bulk, by default 50 at a time. Prints out a zaddr one per lin
 
 Example:
 
-    ./contrib/gen-zaddrs.pl     # generate 50 zaddrs
-    ./contrib/gen-zaddrs.pl 500 # generate 500 zaddrs
+    ./contrib/scripts/gen-zaddrs.pl     # generate 50 zaddrs
+    ./contrib/scripts/gen-zaddrs.pl 500 # generate 500 zaddrs
 
 
 ## Wallet Tools

contrib/scripts/sdl_checkpoints.pl

@@ -10,7 +10,7 @@ use warnings;
 use strict;
 
 # call this script like this to generate checkpoints for a HAC such as DragonX:
-# CLI=./src/dragonx-cli ./contrib/sdl_checkpoints.pl ...
+# CLI=./src/dragonx-cli ./contrib/scripts/sdl_checkpoints.pl ...
 
 my $hush    = $ENV{CLI} || "./src/hush-cli";
 my $gethash = "$hush getblockhash";