`<cstdlib>` was not being included, so malloc and free were only declared
via transitive includes. Some includes changed in the latest libc++-21
build which broke fmt.
Also changed `malloc`/`free` to `std::malloc` and `std::free`, as
putting those symbols in the global namespace is optional for the
implementation when including `<cstdlib>`.
- Add double_type and float_type to scan_type enum
- Add double* and float* pointers to scan_arg union
- Add constructors for double and float scan arguments
- Add switch cases for double and float types in visit()
- Implement basic read() functions for floating-point parsing
This partially resolves the TODO comment 'more types' in scan.h by adding
support for the two most commonly needed floating-point types.
* fix: avoid an implicit cast
The "1" used for the bitshift is treated as int, and this causes an
implicit conversion to `UInt` when performing the logical and.
Explicitly casting the number to `UInt` avoids the warning.
* fix: avoid implicit conversions for indices
Some indices in `include/fmt/base.h` are expressed as `int` types, which
causes an implicit conversion to a `size_t` when they are actually used
as index. Explicitly casting the value avoids the warning.
* fix: avoid an implicit conversion using size_t
The number of bits is used to express the size of a buffer. Using an
`int` causes an implicit conversion warning, let's use a `size_t` which
is the right type for the job.
MSVC hints with:
```
fmt\include\fmt\format-inl.h(26): warning C5244: '#include <locale>' in the purview of module 'fmt' appears erroneous. Consider moving that directive before the module declaration, or replace the textual inclusion with 'import <locale>;'.
```
Then fails the build with `type redefinition`.
