ARROW-2252: [Python] Create buffer from address, size and base

[xhochy]

Usage with Arrow Java vectors:

import jpype
import numpy as np
import pyarrow as pa
import sys

# Start JVM with Arrow and all of its dependencies.
jpype.startJVM(getDefaultJVMPath(), "-Djava.class.path=arrow-tools-0.9.0-SNAPSHOT-jar-with-dependencies.jar")

# Create vector
ra = jpype.JPackage("org").apache.arrow.memory.RootAllocator(sys.maxsize)
uint1 = jpype.JPackage("org").apache.arrow.vector.UInt1Vector("int", ra)
uint1.allocateNew(128)
for i in range(128):
    uint1.setSafe(i, i)
uint1.setValueCount(128)

# Access it in Python
addr = uint1.getDataBuffer().unwrap().memoryAddress()
size = uint1.getDataBuffer().unwrap().capacity()
fb = pa.ForeignBuffer(addr, size, n)
np.asarray(fb)

Output (as expected):

array([  0,   1,   2,   3,   4,   5,   6,   7,   8,   9,  10,  11,  12,
        13,  14,  15,  16,  17,  18,  19,  20,  21,  22,  23,  24,  25,
        26,  27,  28,  29,  30,  31,  32,  33,  34,  35,  36,  37,  38,
        39,  40,  41,  42,  43,  44,  45,  46,  47,  48,  49,  50,  51,
        52,  53,  54,  55,  56,  57,  58,  59,  60,  61,  62,  63,  64,
        65,  66,  67,  68,  69,  70,  71,  72,  73,  74,  75,  76,  77,
        78,  79,  80,  81,  82,  83,  84,  85,  86,  87,  88,  89,  90,
        91,  92,  93,  94,  95,  96,  97,  98,  99, 100, 101, 102, 103,
       104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116,
       117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127], dtype=int8)

[wesm]

+1. When we document this, we should perhaps indicate that the API is experimental