The MMU TLB still requires a lot of checks #20

With the current implementation of the TLB, only page table walks get cached, the checking whether the PTE is valid is done every time. This can be mitigated by:

• Caching the access type together with the translated address, this allows skipping the check for allowed accesses
• Also caching the privilege mode, for which a given access occurred
• Flushing relevant parts of the TLB when mstatus updates, in particular the SUM, MPRV and MXR bits