The initial rationale behind pushing all files into a subdirectory was keeping
the repository root clean, however there's probably better, and move meaningful
ways of doing so (splitting `engine`, `context`, and `value` to separate
packages, moving all `.c` files into the `src` directory etc.
This commit contains an initial version of variable bindings
to a context, along with tests. Currently supported types are
integers, floating point numbers, and strings.