Kernel: Use bzero instead of loop
CircBuffer: Add peek()
Endstops: Fix whitespace
utils: Make string utils const where possible

This will help prevent unnecessary allocations
SerialConsole: reserve a reasonably sized buffer
Merge pull request #83 from bgamari/fixes

Some of these are small changes extracted from @triffid's std::string -> char* conversion which are worth on their own. Each patch is small, self-contained, and hopefully pretty non-controversial.