Place some CODE: chunks inside curly brackets so that the declarations

Place some CODE: chunks inside curly brackets so that the declarations
they begin with are at the start of the block.  This avoid mixed
code/declaration (initially diagnosed by a user with a Windows C
compiler, and tested using GCC with "-ansi -pedantic").  The "correct"
solution is to use INIT: clauses in the XS file, but they don't seem
to work: the code in them is slapped down right next to the CODE:, so
declarations are not acceptable there either.