Replace bitops module with pure-Lua isbitset function

[RiskoZoSlovenska]

This PR replaces the bitops module — used only to check whether certain bits are set — with a pure-Lua function that does just that.

I've benchmarked this change:
With bitops (current behaviour):

Lua 5.2: 0.1465 ± 0.00022
Lua 5.4: 0.1287 ± 0.00023
LuaJIT: 0.0925 ± 0.00014

With isbitset (this PR):

Lua 5.2: 0.1518 ± 0.00029
Lua 5.4: 0.1296 ± 0.00021
LuaJIT: 0.0935 ± 0.00016

The benchmark

Using vips 8.15.1, running on an Intel i3-4010U.

local vips = require("vips")
local Image = vips.Image

local hasBit = pcall(require, "vips.bitops")
print(hasBit and "Using bitops" or "Using custom")

local origImg = Image.black(500, 500)

local samples = 1000
local iter = 500

local sum = 0
local times = {}

for i = 1, samples do
	local img = origImg

	local start = os.clock()
	for _ = 1, iter do
		img = img:sin()
	end
	local stop = os.clock() - start
	collectgarbage()
	collectgarbage()

	times[i] = stop
	sum = sum + stop
end

local avg = sum / samples
local diffsum = 0
for _, time in ipairs(times) do
	diffsum = diffsum + (avg - time)^2
end

local stdev = math.sqrt(diffsum / (samples - 1))

print("Average: " .. avg)
print("Stdev: " .. stdev)
print("Sterr: " .. stdev / math.sqrt(samples))

Calling operations is only 1-4% slower; such slowdown will most likely not be noticeable in practice.

[rolandlo]

Thanks a lot for the PR!

Here are my results: With libvips 8.15.1 on a Intel i7-1165G7 (on Linux)

With bitops:

Lua 5.4:  0.03824 ± 0.00100
Lua 5.3:  0.03920 ± 0.00191
Lua 5.2:  0.04106 ± 0.00240
Lua 5.1:  0.03863 ± 0.00208
LuaJIT:   0.02207 ± 0.00193

With isbitset:

Lua 5.4:  0.03624 ± 0.00281
Lua 5.3:  0.03932 ± 0.00349
Lua 5.2:  0.03953 ± 0.00121
Lua 5.1:  0.03877 ± 0.00137
LuaJIT:   0.02069 ± 0.00076

So the performance is completely fine.