remove bytecode El instruction, rename El2 to El

this commit just updates the bytecode; compiler proofs etc. not touched.
update compiler implementation to not use old El instruction

proofs not updated
update compile_varref_thm
update VfromListCode_correct
update prim1_to_bc_thm
update jmpbc_thm
update compile_val
update compile_print_err_thm
Merge branch 'master' into El2

to get fix for change in fmap compset
failed attempt to prove El implementation correct

I don't know how to get it to say that x2 must be the second thing on
the stack and therefore is a block. It seems like the "prepare" and
"SPEC_WEAKEN" rules don't do quite the right thing here...
implementation of El(2) fixed
LengthBlock implemented in x64
Merge remote-tracking branch 'origin/El2'