Add SourceLocation for parsing error & debug

[vlopes11]

Currently, we display a parsing error based on the line number of a compilation. This is introduced by #854

However, this is very simplistic and we might need a better representation that is flexible enough for advanced functionality, such as source mapping.

It is a common standard to use as location a combination of path, line, column. These 3 combined can uniquely map to a string that can be displayed to the user, to give him feedback.

One possibility is to have the following struct:

pub struct SourceLocation {
    pub path: String,
    pub line: u32,
    pub column: u32,
}

However, for the parsing process, this SourceLocation would exist for every Token. This might become very expensive for large compilations as we would greatly increase the memory requirements with the additional path. In fact, this information would be mostly duplicated, as normally we have a couple of files for compilation, and a far greater tokens count than files count. One alternative is to create a bidirectional map path <-> id, and use this id as part of the source location.

pub struct SourceLocation {
    pub path_id: usize,
    pub line: u32,
    pub column: u32,
}

This would introduce the requirement to have a common context for compilation that would hold the bidirectional map. Such context could look like

pub struct SourcePathRepository {
    path_ids: BTreeMap<String, usize>,
    paths: Vec<String>,
}

The path_ids is an efficient lookup to avoid duplicated paths, and paths would hold the indexes defined by the ids. This way, we can efficiently append new paths, as we would first lookup the path_ids and insert only if it doesn't exist, and we have O(1) to return a path, given an id.

There is also another possibility to further improve this context. The SourceLocation assumes the path exists, but it might be that it will not always be readily available. This might happen if we compile a source with a bundled, pre-compiled library. We could have a set of predefined sources that will be available for any application that will read these source locations and display them to the user:

pub enum Source {
    StandardLibrary {
        module: String,
        version: String,
    },
    AbsolutePath(String),
    RawContents(String),
}

And replace the simple string by this enum

pub struct SourcePathRepository {
    path_ids: BTreeMap<Source, usize>,
    paths: Vec<Source>,
}

This SourcePathRepository can be easily extended into a solution that will support source mapping for debug purposes.

Originally posted by @bobbinth in #854 (comment)

[bobbinth]

I agree with the above, though I think the should start simple and implement SourceLocation just as

pub struct SourceLocation {
    pub line: u32,
    pub column: u32,
}

Once this is working, we can add path_id to SourceLocation. In our case, path would probably be a logical path to a module (rather than a file path) - e.g., std::math::u64 - and the mapping of logical paths to files would need to be provided separately.

[bobbinth]

Closed by #865 and #861.