Long pattern strings

This pull request adds two configure options via environment variables:

* `TRE_EXPRESSION_STACK_SIZE_IN_BYTES`
* `TRE_EXPRESSION_RING_BUFFER_SIZE_IN_BYTES`

This allows the user to compile tre with much large stack and ring buffer sizes, if needed to parse very large pattern strings.

The expression ring buffer is still statically allocated.

A `PACKAGING.md` file is also added with instructions on how to build a distribution package.

(I'm using tre in a project that needs fuzzy matching with very large pattern strings - hence this changeset.)