Use appropriate units when printing wallet balances.

[lemmih]

Issue summary

Right now we always print wallet balances with six fractional digits. Example:

$ ./target/release/forest-cli wallet list
Address                                   Default Balance
f1ahjaxhd4w4qu47t2kasdescbttcgv2ha2doaney           0.000000 FIL
f1necwjawankvg7yyszvzm4oqntlculkfklyzug2q  X        0.000000 FIL
f1phxedvr5653h7gvf7jfg6plrbcgyjl2lkjlxhgi           0.000000 FIL
f1uatkwrkzfn5yuw67sxoosh37rujleu6xvb2dhwq           100.000000 FIL

It would improve the user experience if we default to fewer digits but use appropriate units. For example, someone has 0.001 FIL then it should be printed out as "1 milli FIL" or "1 mFIL". We should also limit the output to, say, 4 significant digits. This can be overridden by a flag, --exact-balance. Additionally, the --fixed-unit flag should force the balance to be in FIL.

Tasks:

• Use 4 significant digits.
• Use appropriate units (FIL, milli FIL, atto FIL, etc).
• Add --exact-balance flag.
• Add --fixed-unit flag.
• Inlcude ~ when the printed balance is not exact.

Example output:

$ ./target/release/forest-cli wallet list
Address                                   Default Balance
f1ahjaxhd4w4qu47t2kasdescbttcgv2ha2doaney           0 FIL
f1necwjawankvg7yyszvzm4oqntlculkfklyzug2q  X        8 atto FIL
f1phxedvr5653h7gvf7jfg6plrbcgyjl2lkjlxhgi           ~500.2 milli FIL
f1uatkwrkzfn5yuw67sxoosh37rujleu6xvb2dhwq           100 FIL

$ ./target/release/forest-cli wallet list --exact-balance
Address                                   Default Balance
f1ahjaxhd4w4qu47t2kasdescbttcgv2ha2doaney           0 FIL
f1necwjawankvg7yyszvzm4oqntlculkfklyzug2q  X        8 atto FIL
f1phxedvr5653h7gvf7jfg6plrbcgyjl2lkjlxhgi           500.236 milli FIL
f1uatkwrkzfn5yuw67sxoosh37rujleu6xvb2dhwq           100 FIL

$ ./target/release/forest-cli wallet list --fixed-unit
Address                                   Default Balance
f1ahjaxhd4w4qu47t2kasdescbttcgv2ha2doaney           0 FIL
f1necwjawankvg7yyszvzm4oqntlculkfklyzug2q  X        ~0 FIL
f1phxedvr5653h7gvf7jfg6plrbcgyjl2lkjlxhgi           ~0 FIL
f1uatkwrkzfn5yuw67sxoosh37rujleu6xvb2dhwq           100 FIL

$ ./target/release/forest-cli wallet list --fixed-unit --exact-balance
Address                                   Default Balance
f1ahjaxhd4w4qu47t2kasdescbttcgv2ha2doaney           0 FIL
f1necwjawankvg7yyszvzm4oqntlculkfklyzug2q  X        0.000000000000000008 FIL
f1phxedvr5653h7gvf7jfg6plrbcgyjl2lkjlxhgi           0.500236 FIL
f1uatkwrkzfn5yuw67sxoosh37rujleu6xvb2dhwq           100 FIL

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