Add MIPS pushstr_array

Example resulting state:

$ shellcraft mips.pushstr_array '$t1' '["hello", "world", "boss!"]' -d --after
...
pwndbg> telescope $t1 4
00:0000| t1 sp  0x76ffec7c --> 0x76ffec8c <-- 'hello'
01:0004|        0x76ffec80 --> 0x76ffec92 <-- 'world'
02:0008|        0x76ffec84 --> 0x76ffec98 <-- 'boss!'
03:000c|        0x76ffec88 <-- 0x0

pwnlib/shellcraft/templates/mips/pushstr_array.asm

@@ -0,0 +1,38 @@
+<% from pwnlib.shellcraft import mips %>
+<%docstring>
+Pushes an array/envp-style array of pointers onto the stack.
+
+Arguments:
+    reg(str):
+        Destination register to hold the pointer.
+    array(str,list):
+        Single argument or list of arguments to push.
+        NULL termination is normalized so that each argument
+        ends with exactly one NULL byte.
+</%docstring>
+<%page args="reg, array"/>
+<%
+if isinstance(array, (str)):
+    array = [array]
+
+array_str = ''
+
+# Normalize all of the arguments' endings
+array      = [arg.rstrip('\x00') + '\x00' for arg in array]
+array_str  = ''.join(array)
+
+word_size = 4
+offset = len(array_str) + word_size
+
+%>\
+    /* push argument array ${repr(array)} */
+    ${mips.pushstr(array_str)}
+    ${mips.mov(reg, 0)}
+    ${mips.push(reg)} /* null terminate */
+% for i,arg in enumerate(reversed(array)):
+    ${mips.mov(reg, offset + word_size*i - len(arg))}
+    add ${reg}, $sp
+    ${mips.push(reg)} /* ${repr(arg)} */
+    <% offset -= len(arg) %>\
+% endfor
+    ${mips.mov(reg,'$sp')}