cgen,pref: add -coverage support

[felipensp]

Enable using v -cov output-file run file.v or v -coverage output-file run file.v.

Currently it is working with one file only.

[spytheman]

Excellent work.

[spytheman]

@felipensp, I've tested running this:

if true {
	println('branch 1')
	if true {
		println('branch 1.1')
		if true {
			println('branch 1.1.1')
		} else {
			println('branch 1.1.2')
		}
	} else {
		println('branch 1.2')
		if true {
			println('branch 1.2.1')
		} else {
			println('branch 1.2.2')
		}
	}
} else {
	println('branch 2')
	if true {
		println('branch 2.1')
		if true {
			println('branch 2.1.1')
		} else {
			println('branch 2.1.2')
		}
	} else {
		println('branch 2.2')
		if true {
			println('branch 2.2.1')
		} else {
			println('branch 2.2.2')
		}
	}
}

with: v -coverage - run branches.v, and I got:

#0 13:07:08 ^ v_coverage /v/oo>v -coverage - run branches.v
branch 1
branch 1.1
branch 1.1.1
V coverage
--------------------------------------------------
[   0.00%]    0 /   98 | /space/v/oo/vlib/strings/builder.c.v
[   0.00%]    0 /   43 | /space/v/oo/vlib/strings/similarity.v
[   0.00%]    0 /   18 | /space/v/oo/vlib/strings/strings.c.v
[   0.00%]    0 /   59 | /space/v/oo/vlib/strings/strings.v
[   0.00%]    0 /  152 | /space/v/oo/vlib/math/bits/bits.v
[   0.00%]    0 /    0 | /space/v/oo/vlib/math/bits/bits_tables.v
[   0.00%]    0 /    8 | /space/v/oo/vlib/math/bits/unsafe_bits.v
[   0.00%]    0 /  191 | /space/v/oo/vlib/strconv/atof.c.v
[   0.00%]    0 /   60 | /space/v/oo/vlib/strconv/atofq.c.v
[   0.00%]    0 /  104 | /space/v/oo/vlib/strconv/atoi.v
[   0.00%]    0 /  176 | /space/v/oo/vlib/strconv/f32_str.c.v
[   0.00%]    0 /  198 | /space/v/oo/vlib/strconv/f64_str.c.v
[   0.00%]    0 /    0 | /space/v/oo/vlib/strconv/f64_str.v
[   0.00%]    0 /   13 | /space/v/oo/vlib/strconv/format.v
[   0.00%]    0 /  269 | /space/v/oo/vlib/strconv/format_mem.c.v
[   0.00%]    0 /    4 | /space/v/oo/vlib/strconv/ftoa.c.v
[   0.00%]    0 /   38 | /space/v/oo/vlib/strconv/number_to_base.c.v
[   0.00%]    0 /    0 | /space/v/oo/vlib/strconv/tables.v
[   0.00%]    0 /  166 | /space/v/oo/vlib/strconv/utilities.c.v
[   0.00%]    0 /   98 | /space/v/oo/vlib/strconv/utilities.v
[   0.00%]    0 /  433 | /space/v/oo/vlib/strconv/vprintf.c.v
[   0.00%]    0 /    9 | /space/v/oo/vlib/builtin/array.v
[   3.36%]    4 /  119 | /space/v/oo/vlib/builtin/array_d_gcboehm_opt.v
[   0.00%]    0 /    4 | /space/v/oo/vlib/builtin/backtraces.c.v
[   0.00%]    0 /   21 | /space/v/oo/vlib/builtin/backtraces_nix.c.v
[  18.48%]   39 /  211 | /space/v/oo/vlib/builtin/builtin.c.v
[   0.00%]    0 /   28 | /space/v/oo/vlib/builtin/builtin.v
[  14.29%]    1 /    7 | /space/v/oo/vlib/builtin/builtin_d_gcboehm.c.v
[  33.33%]    2 /    6 | /space/v/oo/vlib/builtin/builtin_nix.c.v
[  20.00%]    3 /   15 | /space/v/oo/vlib/builtin/cfns_wrapper.c.v
[   0.00%]    0 /    2 | /space/v/oo/vlib/builtin/chan.v
[   0.00%]    0 /   11 | /space/v/oo/vlib/builtin/character_inout.c.v
[   0.00%]    0 /   69 | /space/v/oo/vlib/builtin/float.c.v
[   0.00%]    0 /  207 | /space/v/oo/vlib/builtin/int.v
[   0.00%]    0 /   34 | /space/v/oo/vlib/builtin/map.c.v
[   0.00%]    0 /  306 | /space/v/oo/vlib/builtin/map.v
[   0.00%]    0 /   38 | /space/v/oo/vlib/builtin/map_d_gcboehm_opt.v
[   0.00%]    0 /    3 | /space/v/oo/vlib/builtin/option.c.v
[   0.00%]    0 /    8 | /space/v/oo/vlib/builtin/option.v
[   0.00%]    0 /   15 | /space/v/oo/vlib/builtin/result.v
[   0.00%]    0 /   24 | /space/v/oo/vlib/builtin/rune.v
[   0.00%]    0 /  206 | /space/v/oo/vlib/builtin/sorted_map.v
[   0.00%]    0 /  951 | /space/v/oo/vlib/builtin/string.v
[   0.00%]    0 /    9 | /space/v/oo/vlib/builtin/string_charptr_byteptr_helpers.v
[   0.00%]    0 /  364 | /space/v/oo/vlib/builtin/string_interpolation.v
[   0.00%]    0 /   18 | /space/v/oo/vlib/builtin/utf8.c.v
[   0.00%]    0 /   69 | /space/v/oo/vlib/builtin/utf8.v
[  54.55%]    6 /   11 | branches.v
--------------------------------------------------
Total coverage: 48 files, 1.12% coverage
#0 13:07:41 ^ v_coverage /v/oo>

Why does it print all the .v files for all modules (used in the program), given that for most programs, most of their functionality, will not be used?

I think that skipping all the ones, for which there is 0 coverage will be more useful and more compact.