support scientific notation when parsing numbers

[kmitchener]

select 1e3 currently identifies 1 as a number and e3 as alias. Instead, it should return 1e3 as part of the string Number.

select 1e3; -- returns now e3 as identifier and 1 as number, but should return 1e3 as number
select 1e3a; -- should return 1e3 as number, and a as identifier
select 1e; -- should return 1e as the number (currently returns 1 as number, e as identifier)
select 1e.5; -- correctly returns syntax error
select .5e2; -- should return .5e2 as number, but currently returns .5 as number, e2 as identifier

Since the user of the library will get back a string representing the number anyway and must parse it, we could recognize the e notation and leave it as part of the string in Number.

For example, Rust can parse scientific notation to f64:

    let v = ["1", "1e3", "1e", ".5e2", "1e-1"];
    for s in v {
        let _ = s
            .parse::<i64>()
            .map(|i| println!("parsed {} as i64: {}", s, i))
            .or_else(|_| {
                s.parse::<f64>()
                    .map(|f| println!("parsed {} as f64: {}", s, f))
            })
            .map_err(|_| println!("couldn't parse {}", s));
    }

parsed 1 as i64: 1
parsed 1e3 as f64: 1000
couldn't parse 1e
parsed .5e2 as f64: 50
parsed 1e-1 as f64: 0.1

Only problem is that 1e parses to 1 in Postgres, but it wouldn't in this case without special handling by the parsers. In any case, this is an argument in favor of returning scientific notation as part of the existing Number enum value.

[kmitchener]

The Hive dialect in sqlparser-rs lets it consider the 1e3 in select 1e3 as an identifier. Is that actually true? @andygrove, do you have access to a Hive SQL system and can confirm that behavior?

[kmitchener]

1E+20 is a valid form as well.