speed up import of json reports with RcppSimdJson::fload

[e-kotov]

Hi @JohnPalmer , I have replaced the jsonlite::fromJSON() with substantially faster RcppSimdJson::fload() (see https://github.com/eddelbuettel/rcppsimdjson ) to speed up the mosquitoR::get_malert_data(). I have not measured, but it feels 2-3 times faster and will get more noticeable with every additional year of data.

One caveat, is that the flattening strategy of RcppSimdJson::fload() is different from jsonlite::fromJSON(), which results in a slightly different data structure of the output tibble. I don't know if this change would break any of the existing workflows.

[e-kotov]

I will investigate the failing tests in a few days.