feat: print opt programs wrapped in func.func

[alexkeizer]

This changes the opt output so that the result is now wrapped in a func.func instead of a builtin.module.
NOTE: I literally only changed the latter string into the former, please do check if the result is actually a legal func.func, I don't know what the proper syntax ought to be.

[github-actions]

bv_decide solved 0 theorems.
bitwuzla solved 0 theorems.
bv_decide found 0 counterexamples.
bitwuzla found 0 counterexamples.
bv_decide only failed on 0 problems.
bitwuzla only failed on 0 problems.
both bitwuzla and bv_decide failed on 0 problems.
In total, bitwuzla saw 0 problems.
In total, bv_decide saw 0 problems.
ran rg 'LeanSAT provided a counter' | wc -l, this file found 0, rg found 0, SUCCESS
ran rg 'Bitwuzla provided a counter' | wc -l, this file found 0, rg found 0, SUCCESS
ran rg 'LeanSAT proved' | wc -l, this file found 0, rg found 0, SUCCESS
ran rg 'Bitwuzla proved' | wc -l, this file found 0, rg found 0, SUCCESS
The InstCombine benchmark contains 4520 theorems in total.
Saved dataframe at: /home/runner/work/lean-mlir/lean-mlir/bv-evaluation/raw-data/InstCombine/instcombine_ceg_data.csv
all_files_solved_bitwuzla_times_stddev avg: nan | stddev: nan
all_files_solved_bv_decide_times_stddev avg: nan | stddev: nan
all_files_solved_bv_decide_rw_times_stddev avg: nan | stddev: nan
all_files_solved_bv_decide_bb_times_stddev avg: nan | stddev: nan
all_files_solved_bv_decide_sat_times_stddev avg: nan | stddev: nan
all_files_solved_bv_decide_lratt_times_stddev avg: nan | stddev: nan
all_files_solved_bv_decide_lratc_times_stddev avg: nan | stddev: nan
mean of percentage stddev/av: nan%

[github-actions]

bv_decide solved 0 theorems.
bitwuzla solved 0 theorems.
bv_decide found 0 counterexamples.
bitwuzla found 0 counterexamples.
bv_decide only failed on 0 problems.
bitwuzla only failed on 0 problems.
both bitwuzla and bv_decide failed on 0 problems.
In total, bitwuzla saw 0 problems.
In total, bv_decide saw 0 problems.
ran rg 'LeanSAT provided a counter' | wc -l, this file found 0, rg found 0, SUCCESS
ran rg 'Bitwuzla provided a counter' | wc -l, this file found 0, rg found 0, SUCCESS
ran rg 'LeanSAT proved' | wc -l, this file found 0, rg found 0, SUCCESS
ran rg 'Bitwuzla proved' | wc -l, this file found 0, rg found 0, SUCCESS
The InstCombine benchmark contains 4520 theorems in total.
Saved dataframe at: /home/runner/work/lean-mlir/lean-mlir/bv-evaluation/raw-data/InstCombine/instcombine_ceg_data.csv
all_files_solved_bitwuzla_times_stddev avg: nan | stddev: nan
all_files_solved_bv_decide_times_stddev avg: nan | stddev: nan
all_files_solved_bv_decide_rw_times_stddev avg: nan | stddev: nan
all_files_solved_bv_decide_bb_times_stddev avg: nan | stddev: nan
all_files_solved_bv_decide_sat_times_stddev avg: nan | stddev: nan
all_files_solved_bv_decide_lratt_times_stddev avg: nan | stddev: nan
all_files_solved_bv_decide_lratc_times_stddev avg: nan | stddev: nan
mean of percentage stddev/av: nan%