rename amount in VaultV3.vy to asset to be consistent with ERC4626 standard

[anticlimactic]

see title:

even if we choose to implement erc4626 separately, having the vault call it "amount" and "shares" vs. erc4626 calling it "asset" and "shares" adds cognitive burden to people reading/reviewing the code.

i propose changing reference to amount to asset in the vault code.

[jmonteer]

I think this makes sense.

I will do it in #30

[jmonteer]

closed by #24 / #30