Structs
A lexer reads a mathematical expression and returns a list of tokens in the
expression.
This allows us to read the expression in a simpler way later when we want
to parse it.
When the expression is malformed, the lexer will return this error.
Enums
A list of all possible identifiers
The kind of a lexer error
Tokens are simple things like numbers, operators, parentheses, and so on.
Type Definitions
A possibly big unsigned rational number