use u64 to store the balance of bill and credit

[imotai]

db3>-$ new-db
 database address                           | transaction id 
--------------------------------------------+----------------------------------------------
 0xa15c58bf6ee75980b753396adb4d878129a47468 | qx7NsYyUOKondVHQjG0/itneIAujJofKsUc39Sy0Bpc= 
db3>-$ show-account
 address                                    | bills        | credits       | storage_used | mutations | session | nonce 
--------------------------------------------+--------------+---------------+--------------+-----------+---------+-------
 0x539a6192e4fcd61f53772a3284c7734bc0f6f1d6 | 0.000000 db3 | 10.000000 db3 | 102.00       | 1         | 0       | 0 
db3>-$ show-state
 name       | state 
------------+---------
 database   | 1 
 collection | 0 
 documemt   | 0 
 account    | 1 
 mutation   | 1 
 session    | 0 
 storage    | 102.00  
db3>-$ 

[codecov]

Codecov Report

Merging #318 (f673c43) into main (44d160b) will decrease coverage by 0.35%.
The diff coverage is 41.83%.

@@            Coverage Diff             @@
##             main     #318      +/-   ##
==========================================
- Coverage   62.34%   61.99%   -0.35%     
==========================================
  Files          56       55       -1     
  Lines        5893     5821      -72     
==========================================
- Hits         3674     3609      -65     
+ Misses       2219     2212       -7     

Flag	Coverage Δ
rust	61.99% <41.83%> (-0.35%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/base/src/strings.rs	0.00% <0.00%> (ø)
src/cmd/src/command.rs	64.08% <0.00%> (-3.56%)	⬇️
src/node/src/auth_storage.rs	0.82% <0.00%> (+0.01%)	⬆️
src/node/src/json_rpc_impl.rs	0.00% <0.00%> (ø)
src/node/src/node_storage.rs	0.00% <0.00%> (ø)
src/node/src/storage_node_impl.rs	0.59% <0.00%> (-0.04%)	⬇️
src/storage/src/db_store.rs	89.50% <ø> (ø)
src/types/src/bill_key.rs	100.00% <ø> (ø)
src/types/src/lib.rs	75.00% <ø> (-5.00%)	⬇️
src/sdk/src/store_sdk.rs	85.04% <100.00%> (+0.78%)	⬆️
... and 5 more

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more