Replace bignumber.js with big.js

[gndelia]

This PR replaces bignumber.js with big.js. The goal is to reduce the bundle size, as big.js is significantly smaller than bignumber.js

See the bundle size for [email protected]:

From the diagram, we can see that bignumber.js takes up most of the bundle size

If we compare bignumber.js with big.js, we see the latter is smaller, while having most of the same functionality (or we can get by with some calls to get the same functionality)

BigNumber.js	Big.js

Tests pass so I expect it to have the same functionality.

Description commit by commit:

• df0c59b Removes an incorrect exports (There's no contract in this package ! It must have been a copy-paste error)
• b417f2b Replaces bignumber.js with big.js
• 115e3d8 Adds the bundle mingzipped size badge into the README
• ebea7b0 bumps the version.

Note I believe this should be a breaking change because while the output should stay the same, the input used to accept a BigNumber.js instance, which is no longer accepted. We use BigSource now.