Introduce compilation mode

[Schaeff]

zokrates check -i foo.zok --mode lib

Checks the file up to semantics, not requiring an entry point.

[dark64]

This might be reachable when called from zokrates-js because the config is passed by the caller:

https://github.com/Zokrates/ZoKrates/pull/988/files#diff-31d8ede13068a8e0e79ec05ff89244788c8903841d377659d712b5d2057e9ea6R109

If my understanding is correct, one could set the mode to lib and call compile which would throw here with no clear message.

[dark64]

I think we should use something like this here

#[derive(Debug, Serialize, Deserialize, Clone, Copy, PartialEq)]
pub enum CompileMode {
    #[serde(rename = "bin")]
    Bin,
    #[serde(rename = "lib")]
    Lib
}

so it's a bit cleaner in js: mode: "lib" instead of mode: "Lib". It's also possible to put #[serde(rename_all = "lowercase")] before the enum definition.

We should also update typescript type information file:
https://github.com/Zokrates/ZoKrates/blob/add-compiler-mode-flag/zokrates_js/index.d.ts#L12

export type CompileMode = "bin" | "lib";

export interface CompileConfig {
   allow_unconstrained_variables?: boolean,
   isolate_branches?: boolean,
   mode?: CompileMode
}