Parsa is a functional combinator parsing library. It focuses on concise error handling, modularity, and reusability,
as well as 0-copy string consumption and backtracking.

# Examples
Lets parse the string "var = 123". Note that examples will be using `thiserror` to minimize boilerplate.
First, our struct and error type:
```rust, ignore
use parsa::{ParserString, Parsable};
use parsa::builtins::*;
use thiserror::Error; //simple errors
use parsa::nevermore::FromNever; //From<Infallible> conversion
use std::num::ParseIntError;

struct Var {
    name: String,
    val: i32,
}
#[derive(Debug, Clone, Error, FromNever)]
enum VarErr {
    #[error("missing var name")]
    Empty(#[from] WordErr),
    #[error("missing \"=\"")]
    MissingEqual(#[from] TakeErr),
    #[error("error parsing number: {0}")]
    NumberParse(#[from] ParseIntError),
}
impl Parsable for Var {
    type Err = VarErr;
    fn parse(s: &mut ParserString) -> Result<Self, Self::Err> {
        todo!()
    }
}
```
The first thing we need to do is parse the name of the variable, which we can do with `word`.
We also want to get rid of whitespace, so we can use `whitespace` with the 
`after` combinator, as we dont care about its output.
```rust, ignore
let name = word
    .convert_err::<VarErr>() //explicitly set our target error type
                             //not always needed, but usually helps with inference
    .after(whitespace) //whitespace is infallible, so we dont need an explicit variant
                       //in our error type to coerce from it.
    .parse(s)?;
```
Next, we want to just make sure that the `=` sign comes after.
```rust, ignore
let _ = take("=").after(whitespace).parse(s)?; //coerces to MissingEqual
```
Our final step is to get the number. We will use `word` again, but this time map the result.
```rust, ignore
    let val = word
        .convert_err::<VarErr>() //will save headaches
        .and_then(|s| s.parse::<i32>())
        .parse(s)?;
```
And now we can build our struct!
```ignore, rust
Ok(Var { name, val })
```
And because this function has the correct signature, it can be used with any method in `Parser`.
```ignore, rust
let vars: Vec<Var> = Var::parse.many().parse(s)?;
```