Merge pull request #3384 from tmm1/bitsize-pragma

Implement bitsize pragma for bitfields

tests/pragmas/tbitsize.nim

@@ -0,0 +1,22 @@
+discard """
+ccodeCheck: "\\i @'unsigned int flag:1;' .*"
+"""
+
+type
+  bits* = object
+    flag* {.bitsize: 1.}: cuint
+    opts* {.bitsize: 4.}: cint
+
+var
+  b: bits
+
+assert b.flag == 0
+b.flag = 1
+assert b.flag == 1
+b.flag = 2
+assert b.flag == 0
+
+b.opts = 7
+assert b.opts == 7
+b.opts = 9
+assert b.opts == -7