Add `fillv`, `memset` and comparison functions

• Add memset to set all the bytes of a cstruct value efficiently (#49)
• More useful Invalid_argument parameters (#48).
• Fix to_sexp to expose only the current view (#44 from David Kaloper).
• Add compare and equal (#23, #24 and #45 from David Kaloper).
• Add fillv to copy over a list of buffers (from Thomas Leonard).
• Shift to centralised Travis scripts.