SL-16207: Create uniform overload sets for wide-string conversions.

Use new ll_convert_forms() macro in llstring.h to declare, for each
wide-string conversion function of interest, four overloads. The real one, the
nontrivial one, is (const char*, size_t len), implemented in llstring.cpp. Then
(const string&, size_t len), (const char*) and (const string&) are each
trivially implemented with an inline call to (const char*, size_t len).

Notably, we change all S32 len parameters to size_t. Using S32 is old skool.

Tweak each nontrivial implementation in llstring.cpp to accept (const char*,
size_t len) instead of (const string&) with or without explicit length.
Eliminate from llstring.cpp trivial overloads (deriving length from either a
const char* or from a string), since those are now inline in the header.

Of course three of those overloads will be unified once we enable C++17 and
change each relevant parameter to std::string_view, but we're not yet there.
Meanwhile, this suite of overloads minimizes, to the best of our ability, new
string allocations solely for parameter passing. And use of a macro means we
need only change the macro once we get std::string_view.

We take this step because some use cases require (const char*), some require
(const string&, size_t len), others (const char*, size_t len) ... We were
missing some key overloads, and had to work around them by instantiating new
string objects (necessitating both allocation and character copying) just to
pass the desired parameter. Using the macro ensures this consistent set of
overloads for every wide-string conversion function.

Additionally, knowing that the ugly-name overloads exist, ll_convert_forms()
implicitly defines corresponding ll_convert<TARGET>() overloads.

Streamline declarations of utf16str_to_wstring(), wstring_to_utf16str(),
utf8str_to_utf16str(), utf16str_to_utf8str(), utf8str_to_wstring(),
wstring_to_utf8str(), ll_convert_wide_to_wstring() and
ll_convert_wstring_to_wide() using ll_convert_forms().

Use corresponding new ll_convert_cp_forms() macro to declare consistent
overloads for conversion functions accepting an optional unsigned int
code_page parameter. We used to delegate to the .cpp file the implementation
of each overload accepting code_page so llstring.h need not include the
Windows header defining the CP_UTF8 default; this is more simply accomplished
by introducing a small ll_wstring_default_code_page() function to retrieve it
from the .cpp file. That lets us specify the code_page parameter as optional,
using that function as its default value.

Use ll_convert_cp_forms() to streamline declarations of
ll_convert_wide_to_string() and ll_convert_string_to_wide().

Introduce real implementations of ll_convert_wide_to_wstring() and
ll_convert_wstring_to_wide(). The previous implementations merely copied
individual characters, which is wrong: when we convert UTF16LE to UTF32, we
can and should fold multi-character UTF16LE encodings to the corresponding
single UTF32 character. The real implemenations leverage our awareness that
both llutf16string and Windows std::wstring (either variant) use UTF16LE
encoding, so we can reuse the corresponding llutf16string conversions.

Introduce generic ll_convert_length() function, specialized as either
std::strlen() or std::wcslen() depending on parameter type. (Even if
std::wcslen() is derived from classic C, why doesn't the C++ standard library
define a std::strlen(const wchar_t*) overload to call it?)

Fix ll_convert_alias()'s ll_convert_impl specialization's operator() to accept
boost::call_traits::param_type, so we can pass (e.g.) const std::wstring& but
also const wchar_t* instead of const wchar_t*&.

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