Fixes#472.
This also partially addresses #195. However, as pointed out in one of
the comment, there's no 'official rule' when it comes to what should
be considered valid escape sequences. Haskell relies mostly on the
AttoParsec library and Rust also has its own set of rules.
This is in particular true for unicode escape sequences, but there is
a common middleground for some usual single character escapes such as
\n or \\. So we now at least support these.
For more complicated escape sequence, please refer to #195 for now and
keep the discussion going there.
Supersedes #35.
The syntax for these elements isn't "set in stone"; in the sense that it is unspecified in [input-output-hk/plutus](https://github.com/input-output-hk/plutus). There's no visible plan from IOG to extend the Haskell parser to support this syntax, though there are samples of imagined syntax in the code. Thus, we can lead the way and simply choose a suitable syntax and let the Haskell implementation align to it later.
This syntax is thus inspired from input-output-hk/plutus' samples, with only a small change: we use `<` and `>` for encapsulating type declaration instead of `(`, `)`. There are already enough parentheses in the UPLC syntax, adding more reduces visibility.
Doing this, I've also added a lot more test cases for the UPLC parser. There could be more, but this is a good start.
Here are some example programs (taken from test cases) utilizing this syntax:
```
(program 0.0.0 (con list<bytestring> [#00, #01]))
```
```
(program 0.0.0
(con pair
<integer, integer>
[14, 42]
)
)
```
```
(program 0.0.0
(con pair<string, list<integer>> ["foo", [14, 42]])
)
```
_(Note that this was mainly done as an exercise to get more familiar with Rust and parts of Aiken.)_
Niels Mündler