Optimise substring functionality in the lexer

[dasunpubudumal]

Currently, since std::string::String does not contain a substring function, we have adhered to a more "unconventional" approach to substring a string. The approach is as follows:

1. Declare a vector Vec<char> and push characters to the vector through an iterator.
2. Use String::from_iter function to convert the vector into a string.

There are other more traditional approaches to this; examples such as Java's String.substring function comes to mind.

The idea behind this issue is to create a custom utility function for substring or bring in a crate and integrate it to the current codebase.