Allow var declaration in header to use in eval blocks

[Avi Sternlieb]

For brainfuck state logging syntax currently used in AviFS/tabl & AviFS/bf-webapp, one should be able to do something like the following:

nzero = tape[i]?tape[i]:'0'
pointer = ptr[i]?' ':'*'

.{zero:>3}{pointer}..

Note that these look like they're only allowing for static vars to be declared, but they're implicitly functions of i & j.

Originally posted by @AviFS in #7 (comment)