Make include guards unique

[elfring]

I find that include guards like "AST_H" and "VM_H" are too short for the safe reuse of your header files (when they belong to an application programming interface).

[sparkprime]

So those files are not part of the API -- the only thing you need to include is lbijsonnet.h which currently has no header guards on account of only containing declarations that can be re-declared without errors. If I need to add header guards I'll use LIBJSONNET_H.

[sparkprime]

I'll just add JSONNET_ to all the others though, it's not like it'll cost us anything.

[elfring]

Thanks for your improvement of include guards.