ranges: Add size() and empty() functions as well as additional constructors

[stotko]

The API of the ranges module, which aims to model the C++20 ranges library in a significantly simplified yet still useful manner, is still rather limited. Add additional constructors to host_range, device_range and transform_range as well as size() and empty() functions and some typedefs. Furthermore, replace the non-const begin() and end() functions with equivalent const versions and implicitly deprecate the non-const ones. This is a small step towards a more useful ranges module.

[codecov]

Codecov Report

Merging #122 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #122      +/-   ##
==========================================
+ Coverage   97.87%   97.92%   +0.04%     
==========================================
  Files          27       28       +1     
  Lines        1648     1685      +37     
==========================================
+ Hits         1613     1650      +37     
  Misses         35       35              

Impacted Files	Coverage Δ
src/stdgpu/impl/ranges_detail.h	100.00% <100.00%> (ø)
src/stdgpu/ranges.h	100.00% <100.00%> (ø)

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