Partially automated with cargo clippy --fix.

…825)

Previously, the constructors had their logic nearly duplicated, one to fit the
const fn context, with minor differences. This change consolidates it back
into a single const fn, with the non-const parts separated out, so that the
parsing logic does not drift unintentionally.

…slash (#826)

Closes #507

Rephrase the comment on `map` field in `Extensions` to make it easier to understand