* Use std::char_traits::length for ansi_color_escape::begin

-Fixes issue #1594 https://github.com/fmtlib/fmt/issues/1594

FMT_DEPRECATED is now defined as FMT_HAS_CPP14_ATTRIBUTE(deprecated), as this attribute was introduced in C++14.

FMT_FALLTHROUGH is now defined as FMT_HAS_CPP17_ATTRIBUTE(fallthrough), as this attribute was introduced in C++17.

FMT_MAYBE_UNUSED is defined as FMT_HAS_CPP17_ATTRIBUTE(maybe_unused), as this attribute was introduced in C++17.

FMT_MAYBE_UNUSED has been applied to fix a couple of -Wunused-member-function warnings from clang.

* Workaround broken [[deprecated]] in PGI compiler
  - similar to Intel and NVCC, add workaround for PGI compiler

FMT_STRING_IMPL has an internal helper named FMT_STRING, however FMT_STRING is also the name of the macro that invokes FMT_STRING_IMPL.

Renaming this helper avoids the appearance of a recursive macro.

grisu_count_digits is only used by grisu_gen_digits, which assigns the unsigned result to a (signed) int.

Although grisu_count_digits always returns a positive integer this keeps its return type in sync with the type its result is assigned to.

* Fix -Wsign-conversion in bigint::subtract_aligned.

n is assigned a size_t, and only used for comparisons with j.

j is assigned 0, compared to n (size_t), and passed to basic_memory_buffer::operator[] (size_t).

* Fix -Wsign-conversion in bigint::assign.

num_bigits is initialised to 0, is only ever incremented, and is passed to basic_memory_buffer::operator[] (size_t) and basic_memory_buffer::resize (size_t).

Previously format.h defined FMT_CLANG_VERSION after including core.h, however core.h tests FMT_CLANG_VERSION when it defines FMT_API.

When building with -Werror,-Wmissing-noreturn clang identifies that assert_fail could be declared with the 'noreturn' attribute.

The current `FMT_FORMAT_AS` macro will make `formatter<Char *>::format`
have the first argument type `const Char *&` which is incorrect an
should be `Char *const &`.  This pull request fixes that by changing the
first argument type in the macro definition body from `const Type &` to
`Type const &`.

* improved use of find

*begin is supposed to be different from '{' when this find is used, so we can avoid checking it.

* add const begin and end overload to buffer

since there is a const overload for data I think there should also be one for begin and end