Remove Default implementation from sysvars

[febo]

Problem

Currently the Rent sysvar includes a default implemention using the Default derive. This makes all values to be set to 0, which differs from the default values used in the SDK.

Solution

This PR adds a manual implementation for the Default trait with the expected default values.

Since the Default implementation is not strictly required for sysvars, this PR removes the requirement from the Sysvar trait and updates the sysvars implementation.

cc: @d0nutptr

[joncinque]

Does this need a Default implementation at all? It's a bit dangerous to have it, especially because we're considering changing it for solana-foundation/solana-improvement-documents#194

I'd probably recommend removing it, but if it is needed, it should match the sdk's version

[febo]

Does this need a Default implementation at all? It's a bit dangerous to have it, especially because we're considering changing it for solana-foundation/solana-improvement-documents#194

I'd probably recommend removing it, but if it is needed, it should match the sdk's version

Good point! At the moment it is needed since the Sysvar trait requires it. But we can change that as well, since it is not really required.

I made the Sysvar change locally and everything else works.

[joncinque]

Looks great!