Fixed small numbers parsing

[slavafomin]

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This PR fixes the issue https://github.com/ethjs/ethjs-unit/issues/6.

Released as: @ton.js/[email protected] for testing and a as quick fix.

[slavafomin]

Solution is pending. Current implementation has some flaws.

[slavafomin]

I've updated number parsing to use Intl for more reliable results, both toString and toFixed give incorrect results for some numbers.

[slavafomin]

Published as @ton.js/[email protected] for testing and as a hotfix.