this includes:
- Data Sources
- Utilities
- Tests

Using -ffast-math is causing linking problems like the one below in the
nightly builds with Clang 7 + GCC 6.2:

CMakeFiles/VecOps.dir/src/TVec.cxx.o: In function `exp':
 /.../include/c++/6.2.0/cmath:246: undefined reference to `__expf_finite'

This is because when linking with -ffast-math, the compiler is expecting
the symbol __expf_finite to be in libm.so, but it is not there:

$ nm -D /lib/libm-2.12.so | grep expf
00012010 W cexpf
00010aa0 W expf
000141d0 W frexpf
00014240 W ldexpf

The vectorized math functions were only introduced in glibc-2.22, so
on a machine with a more recent glibc, however, the symbols are there:

$ nm -D /lib/libm-2.26.so | grep expf
000000000004d5d0 W cexpf
000000000006bbe0 W cexpf128
000000000000f930 W expf
000000000006fb10 W expf128
0000000000050f20 T __expf128_finite
0000000000040de0 T __expf_finite
00000000000489b0 W frexpf
0000000000064080 W frexpf128
0000000000048a60 W ldexpf
00000000000641b0 W ldexpf128

More information: https://sourceware.org/glibc/wiki/libmvec

We would like to use, e.g. decltype(std::sin(std::declval<T>())) to get
the right return type, but the resulting mangled name creates ABI issues
with certain compilers. Since the rules are straightforward at least for
the math functions we intend to wrap, we can use custom rules that yield
the same return types as using decltype to avoid these issues.

This avoids using std::declval<T>() or alternatives like T() or T{},
which do not work with all compilers.

This allows to drop the dependency on ROOT/RStringView.hxx.

This is to avoid incompatibilities with older compilers that may arise
from using std::string_view and ROOT's backport of it.

The interface switched from iterator to const_iterator in C++11,
but GCC 4.8 still uses the old interface and breaks with the error
below if const_iterator is used:

error: no matching function for call to
  ‘std::vector<char, std::allocator<char> >::erase(TVec<char>::const_iterator&)’
   iterator erase(const_iterator pos) { return fData.erase(pos); }
                                                           ^

This reverts commit 7fa206c4.
It introduces a spurious load of libRooFit, visible in the failure of
projectroot.roottest.root.treeproxy.roottest_root_treeproxy_make

this functionality has been removed since there are still compilation errors.

a

[cxxmodules] Fix recent failures due to VecOps and TreePlayer namespace lookups.

to workaround the failing autoloading on Ubuntu.

Constructors and operators of TVec must be defined so that when using
extern templates specializations, all symbols are present in the library.

For rbegin(), rend(), etc, to work, the it++ and it-- operations
must be reversed too.

Compiler does not know how to convert fData (i.e. std::vector<T>) to
a TVec<T>& by itself.

This is following the recent decision by the team to list all files
so that CMake knows when it needs to regenerate the build system due
to added/removed files.