Readability improvements for decode.c:

Several data arrays that are used in only one function have been moved from file scope to the function that uses them.

Updated to use C99 array designators so we can specify values by array index.  This is more readable and less error-prone than using comments to label each value.

There is no functional change for the vast majority of this CL.  The only exception is that we now test for OOM when creating a map.

PiperOrigin-RevId: 475191072