Migrate to u128 balances

The only difficult thing should be to decide whether migration is necessary and to make the migration code. Or just reset the chain.

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With the following initial state:

• 10M members
• 10kG1/member
• 10%/year growth rate

Monetary mass after 100 years is m = 1e6 * 10e3 * 1.1**100.

2**63 / m == 66930 so there would still be enough room for computations.

Given that these predictions are very longterm and unrealistically high, 64 bits seems enough for the foreseeable future.

Some precautions should be taken anyway to avoid overflows in intermediate computations, as G1 approaches 2**32. (e.g. two amounts should NOT be multiplied)

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This issue is still there: https://forum.duniter.org/t/bug-on-transfer-fees-in-substrate-interface-1-4-2/10245

Removing the need for client devs to not use a default value, is it the only motivation?

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