Investigate the feasibility of changing the key scheme for dictionaries

[cspramit]

Investigate the feasibility of changing the key scheme for dictionaries to allow using the key-prefix iterator

[alsrdn]

Did an initial survey on the dictionary items that exist in global state using a snapshot of the mainnet global state from block height 2163794. Here's the summary:

Number of dictionary keys found: 164996
Number of unique seed uref addrs: 1427

Histogram of dictionary lengths in bins of size 100 items (sorted):
0     - 99   : **************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************
100   - 199  : **************************************************************************
200   - 299  : *****************
300   - 399  : ******************************
400   - 499  : ***
500   - 599  : *********
600   - 699  : ***
700   - 799  : *
800   - 899  : ***
1100  - 1199 : ****
1200  - 1299 : *****
1800  - 1899 : *
2100  - 2199 : ***
2200  - 2299 : *
2300  - 2399 : **
2400  - 2499 : **
2500  - 2599 : ****
2600  - 2699 : **
3100  - 3199 : *
5000  - 5099 : *******
5100  - 5199 : **
5800  - 5899 : **
10600 - 10699: *

For readability, number of dictionaries with lengths between 0 - 99 is 1250, for 100 - 199 number is 74.

[rafal-ch]

Moving back to backlog, since this is "nice to have" and Alex needs to focus on the higher prio task: https://github.com/casper-network/roadmap/issues/190