[cmath.syn] Consolidate std namespaces

There is no ordering dependency between the two typedefs
in namespace std, the macros that follow, and teh next
opening of namespace std, so move the two typedefs to
avoid repeatedly opening an closing the namespace.

Note that we could have done this without moving
the typedefs as macros are not bound by namespaces,
but our convention very sensibly avoids confusing
readers by keeping macro definitions outside of
namespaces.

Author: AlisdairM

source/numerics.tex

@@ -9304,11 +9304,6 @@
 \indexlibraryglobal{truncf}%
 \indexlibraryglobal{truncl}%
 \begin{codeblock}
-namespace std {
-  using float_t = @\seebelow@;
-  using double_t = @\seebelow@;
-}
-
 #define @\libmacro{HUGE_VAL}@ @\seebelow@
 #define @\libmacro{HUGE_VALF}@ @\seebelow@
 #define @\libmacro{HUGE_VALL}@ @\seebelow@
@@ -9330,6 +9325,9 @@
 #define @\libmacro{math_errhandling}@ @\seebelow@
 
 namespace std {
+  using float_t = @\seebelow@;
+  using double_t = @\seebelow@;
+
   constexpr @\placeholdernc{floating-point-type}@ acos(@\placeholdernc{floating-point-type}@ x);
   constexpr float               acosf(float x);
   constexpr long double         acosl(long double x);