PERF: cython optimizations
cython was emitting some warnings about untyped array lookups in `tslibs.conversion`; added types to those

`arr.fill(val)` is a python call, while `arr[:]` is a C call, so replaced all of those

modernized an old non-python for-loop, some other small cleanups along the way.

We're running out of available optimizations/cleanups in these files.  One thing I'd be interested in changing (but I'm not confident enough to do) is removing uses of `double` and `double_t`, which I _think_ are equivalent to `float64_t`.  But I'm not sure if that is architecture or platform dependent or something.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23477?src=pr&el=h1) Report
> Merging [#23477](https://codecov.io/gh/pandas-dev/pandas/pull/23477?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/aaaac86ee019675119cb0ae9c3fb7a2b7eef9959?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23477/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23477?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23477   +/-   ##
=======================================
  Coverage   92.23%   92.23%           
=======================================
  Files         161      161           
  Lines       51197    51197           
=======================================
  Hits        47220    47220           
  Misses       3977     3977
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <ø> (ø)` | :arrow_up: |
| #single | `42.27% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23477?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23477?src=pr&el=footer). Last update [aaaac86...a7b9aa0](https://codecov.io/gh/pandas-dev/pandas/pull/23477?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 > We're running out of available optimizations/cleanups in these files.

@jbrockmendel : Gosh, I wonder whose **"FAULT"** that is 😉  > let's make this an AssertionError

Happy to do so, though now that I look closer, isn't this redundant with the check at the beginning of the function? BTW any idea about the double_t thing? what is the problem with double_t ?
this is just float64 > what is the problem with double_t ?  this is just float64

Yah that's the thing I want to double-check before changing all the places where we currently use `double` or `double_t` to `float64_t` thanks!