Add `sexp` decorator for cenum values

Add a sexp optional decorator to cenum to output the values as s-expressions.
This is compatible with the sexplib convention. The syntax is;

cenum foo64 {
  ONE64;
  TWO64;
  THREE64
} as uint64_t(sexp)

And sexp_of_foo64 and foo64_of_sexp functions will also be available.
The representation of the Sexp is the string representation of the enum.

Add `to_bigarray`

Add a (sadly rather slow but sometimes necessary) function to convert a Cstruct back into a Bigarray slice of the right length and offset.

Bug fix for Cstruct.shift

Important bugfix for Cstruct.shift that affected the v1.0.0 release. Use this release in preference.

First stable release

Documentation and bounds checking improvements

More permissive grammar, and variable length buffers

• Trailing semicolons are allowed in cstruct field definitions.
• Buffer elements can be any primitive integer, not just uint8.

bug fixes, Travis and hexdump pretty printing

• Improved ocamldoc for BE/LE modules.
• Add Travis-CI test scripts and fix test.sh script compilation.
• Support int32/int64 constant values in cenum like VAL = 0xffffffffl, useful for 32-bit hosts.
• Check and raise error in case of negative offsets for blits (#4).
• Correctly preserve the sequence after a constant constructor is set during a cenum definition.
• Do not repeat the sizeof_<field> binding for every get/set field (should be no externally observable change).
• Add Cstruct.hexdump_to_buffer to make spooling hexdump output easier.
• Generate hexdump_foo and hexdump_foo_to_buffer prettyprinting functions for a cstruct foo.