[GUI] Support gui.arrow() and gui.arrows()

[archibate]

Related issue = #

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This could be useful when visualizing a vector field, particle velocities and forces.
It's completely based on gui.lines(), so no C++ changes involved. It's just handy for users so they don't have to hand-write that much numpy ad-hoc operations on their own.

To test:

import taichi as ti
import numpy as np

gui = ti.GUI()

orig = np.array([[0.5, 0.3], [0.3, 0.8]], dtype=np.float32)
dir = np.array([[-0.1, 0.4], [0.4, -0.2]], dtype=np.float32)

while gui.running and not gui.get_event(gui.ESCAPE):
    gui.arrows(orig, dir, radius=2, angle=25, tip_scale=0.4)
    #gui.arrow(orig[0], dir[0], radius=2)
    gui.show()

OFT: I assigned a default value title='Taichi' for ti.GUI since thinking about a title is often mind-taking, at least to me.

[codecov]

Codecov Report

Merging #1625 into master will decrease coverage by 22.21%.
The diff coverage is 49.05%.

@@             Coverage Diff             @@
##           master    #1625       +/-   ##
===========================================
- Coverage   67.08%   44.87%   -22.22%     
===========================================
  Files          40       40               
  Lines        5691     5756       +65     
  Branches      993     1004       +11     
===========================================
- Hits         3818     2583     -1235     
- Misses       1699     3019     +1320     
+ Partials      174      154       -20     

Impacted Files	Coverage Δ
python/taichi/__init__.py	0.00% <0.00%> (-75.00%)	⬇️
python/taichi/misc/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
python/taichi/misc/gui.py	0.00% <0.00%> (-23.13%)	⬇️
python/taichi/misc/image.py	41.17% <0.00%> (-15.69%)	⬇️
python/taichi/misc/util.py	17.33% <0.00%> (-27.12%)	⬇️
python/taichi/lang/matrix.py	67.31% <33.33%> (-24.72%)	⬇️
python/taichi/testing.py	76.92% <76.92%> (ø)
python/taichi/lang/__init__.py	55.20% <100.00%> (-25.01%)	⬇️
python/taichi/lang/core.py	0.00% <0.00%> (-100.00%)	⬇️
python/taichi/core/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
... and 29 more

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[k-ye]

LGTM!

[k-ye]

Is this equivalent to transpose?

[archibate]

swapaxes? Yes!