Rename CMake project from stdlib to fortran_stdlib

[awvwgk]

Part 2/3 for #287, status: optional. If you do not like this approach we can do #287 without it.

Changes:

• rename CMake project from stdlib to fortran_stdlib
• reuse the PROJECT_NAME CMake variable instead of using fortran_stdlib directly

Reasoning:

• exported CMake targets usually match the project name (fortran_stdlib seems less likely to clash with something compared to stdlib)
• allows to easily use PROJECT_NAME to write reusable/transferable CMake code for different projects (interesting for fpm generated CMake files as well)
• PROJECT_NAME is accessible in configure_file and can be used to create more general template files