type_traits: Move to public module and add documentation

[stotko]

So far, type_traits only contained types and macros only intended for internal usage. With the addition of remove_cvref, it now contains a publicly facing API. Move it out from the internal implementation directory and add respective documentation for it.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (dc5c353) 97.36% compared to head (fda153f) 97.36%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #379 +/- ## ======================================= Coverage 97.36% 97.36% ======================================= Files 31 31 Lines 2547 2547 ======================================= Hits 2480 2480 Misses 67 67 ``` | [Impacted Files](https://app.codecov.io/gh/stotko/stdgpu/pull/379?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko) | Coverage Δ | | |---|---|---| | [src/stdgpu/impl/memory\_detail.h](https://app.codecov.io/gh/stotko/stdgpu/pull/379?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko#diff-c3JjL3N0ZGdwdS9pbXBsL21lbW9yeV9kZXRhaWwuaA==) | `99.00% <ø> (ø)` | | | [src/stdgpu/impl/unordered\_base.cuh](https://app.codecov.io/gh/stotko/stdgpu/pull/379?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko#diff-c3JjL3N0ZGdwdS9pbXBsL3Vub3JkZXJlZF9iYXNlLmN1aA==) | `100.00% <ø> (ø)` | | | [src/stdgpu/unordered\_map.cuh](https://app.codecov.io/gh/stotko/stdgpu/pull/379?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko#diff-c3JjL3N0ZGdwdS91bm9yZGVyZWRfbWFwLmN1aA==) | `100.00% <ø> (ø)` | | | [src/stdgpu/unordered\_set.cuh](https://app.codecov.io/gh/stotko/stdgpu/pull/379?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko#diff-c3JjL3N0ZGdwdS91bm9yZGVyZWRfc2V0LmN1aA==) | `100.00% <ø> (ø)` | | | [src/stdgpu/vector.cuh](https://app.codecov.io/gh/stotko/stdgpu/pull/379?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko#diff-c3JjL3N0ZGdwdS92ZWN0b3IuY3Vo) | `100.00% <ø> (ø)` | |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.