Add compatibility with pre-ES2016 environments (like some React Native JS compilers) by adding a custom Buffer.subarray polyfill

[overcat]

We need to wait for the new version of js-xdr to be released first.

[Shaptic]

@overcat @stellar/js-xdr@v3.1.1 is out

[overcat]

stellar-base also uses Buffer.subarray

js-stellar-base/src/util/decode_encode_muxed_account.js

Line 109 in d87eb01

ed25519: rawBytes.subarray(0, -8)

[overcat]

Hi @Shaptic, stellar-sdk does not seem to directly use Buffer.subarray, so we do not need to add this polyfill in stellar-sdk for now. We just need to update the version of stellar-base in stellar-sdk.

So next, we need to:

1. Merge this PR
2. Release the new stellar-base
3. Update the stellar-base version in stellar-sdk
4. Release the new stellar-sdk