The default Go stack size limit is insufficient for some highly complex
Verifpal analyses. I have tried to determine whether the stack size was
being reached due to an infinite loop and evidence suggests that this is
not the case. Therefore, the stack size limit is increased from the
default 1GB to 8GB.

Errors must always bubble up from vplogic, with fatal events only
triggered by Verifpal's high-level command-line wrapper.

Make Verifpal logic exportable as a Go library called vplogic, with the Verifpal command acting as a wrapper

Fix race condition that resulted in bug identified by Renaud Lifchitz:
https://lists.symbolic.software/pipermail/verifpal/2020/000263.html

Worth noting that this race condition was not detected by Golang's race
detector despite many attempts.

In order to accomodate the fact that Verifpal will now be translating to
Coq, ProVerif and soon Go, the project structure inside the `internal`
folder is now such that we have Verifpal's code itself and a bunch of
"lib" folders:

- libpeg implements the Verifpal grammar in Peg and generates the
Verifpal model parser.
- libcoq implements the Verifpal Coq library/header.
- libpv implements the Verifpal ProVerif headers.
- libgo will implement the Verifpal Go implementation headers etc.

Whenever a "lib" is compiled, the resulting artifact goes in
internal/verifpal/libname.go.

The Makefile should hopefully compile these libs whenever is necessary
automatically.

Go won't allow struct properties to be marshaled into JSON unless
they (and their subtypes) are exported.

What a nightmare.

This commit introduces a bunch of things:

- Freshness queries:
    Freshness queries are useful for detecting replay attacks.
    In passive attacker mode, a freshness query will check whether
    a value is "fresh" between sessions (i.e. if it has at least
    one composing element that is generated). In active attacker
    mode, it will check whether a value can be rendered "non-fresh"
    (i.e. static between sessions) and successfully used between
    sessions. This is still an experimental feature.
    Actual documentation will land in the Verifpal User Manual.

- Unlinkability queries:
    Totally not there yet, but this commit introduces them to the
    parser and sanity checker. The query itself does not currently
    do anything.

- Phases:
    Phases now work in passive attacker mode. Previously they only
    worked with an active attacker.

A "freshness.vp" test has been added.