Alias reimplementation #711

Created by: ishanjain28

Alias Reimplementation as discussed in #711 (closed)

Changes introduced by this pull request:

• Removed Math Expressions parser from parse_alias.
• Alias reimplemented as a keyword exactly like let or export.

Drawbacks: I have made an assumption in my code that might be considered a drawback.

The assumption is that, Any value stored in alias will always resolve to a Statement::Pipeline, I don't know what should be the appropriate behaviour if it does not resolves to a Statement::Pipeline, So, It silently ignores that PipeItem.

TODOs:

• Tests(I'll add those if and once this PR is approved)

Fixes: #711 (closed)

State: WIP, Everything except for the tests is done.