Token type is the output of tokenizer, a digested interpretation of the source code. A token type is an object including a few meta infos, e.g. whether a token is an assignment operator (isAssign), or the precedence if it is a binary operator (binop). While accessing info from the token type directly is convenient, it introduces extra object access instructions.

In this PR we store the token type as a number. By doing so we can eliminate all the tt.* usage at build time, replacing them to a number, which is similar to how we handle the charCodes.* usage: The babel-plugin-transform-charcodes replaces them by the actual numeric values.

Because the token types are now a continuum of integers, some queries can be simplified to a in-range call if we carefully align the declarations of token types. For example, in this PR, token.isLoop is reimplemented as:

token >= tt._do && token <= tt._while;

the ordinality of integers allow us to reduce multiple branch condition to two branch condition*.

Since we export the tokens via tokTypes, an extra compatibility layer is introduced so the parser can work on internal number token types while emitting the object token types when tokens is enabled.

function inRange(n, start, end) {
  return (n - start) >>> 0 <= end - start
}

But I didn't observe actual performance improvements. Since the JavaScript does not have an unsigned u32 type, the compiler can not actually use u32 here.