The previous function call sequence for ordinary global functions
looked as follows.

1. check whether the function is defined, i.e. whether
   symbol->symbol.gfdef is not NULL
2. set the_env->function to symbol->symbol.gfdef
3. call the function pointer symbol->symbol.gfdef->cfun.entry

This commit implements a performance optimization that enables us to
skip the first step. The basic idea is to replace symbol->symbol.gfdef
with a closure that signals an undefined-function condition.

However, straightforwardly implementing this would have the
disadvantage that it would consume a larger amount of memory for each
symbol without a function definition. To get around this, we reorder
the fields of the ecl_symbol struct such that the symbol can serve as
the function object itself, introducing an entry point that is only
used for undefined functions.

Benchmarking shows an improvement of about 10% in thight loops
compared to the old method.