Experimental support for delayed instantiation of generics

This postpones the semantic pass over the generic's body until
the generic is instantiated. There are several pros and cons for
this method and the capabilities that it enables may still be possible
in the old framework if we teach it a few new trick. Such an attempt
will follow in the next commits.

pros:
1) It allows macros to be expanded during generic instantiation that
will provide the body of the generic. See ``tmacrogenerics``.
2) The instantiation code is dramatically simplified. Dealing with unknown
types in the generic's body pre-pass requires a lot of hacky code and error
silencing in semTypeNode. See ``tgenericshardcases``.

cons:
1) There is a performance penalty of roughly 5% when bootstrapping.
2) Certain errors that used to be detected in the previous pre-pass won't
be detected with the new scheme until instantiation.

tests/compile/tgenericshardcases.nim

@@ -0,0 +1,30 @@
+discard """
+  file: "tgenericshardcases.nim"
+  output: "int\nfloat\nint\nstring"
+"""
+
+import typetraits
+
+proc typeNameLen(x: typedesc): int {.compileTime.} =
+  result = x.name.len
+  
+macro selectType(a, b: typedesc): typedesc =
+  result = a
+
+type
+  Foo[T] = object
+    data1: array[high(T), int]
+    data2: array[1..typeNameLen(T), selectType(float, string)]
+
+  MyEnum = enum A, B, C,D
+
+var f1: Foo[MyEnum]
+var f2: Foo[int8]
+
+static:
+  assert high(f1.data1) == D
+  assert high(f1.data2) == 6 # length of MyEnum
+
+  assert high(f2.data1) == 127
+  assert high(f2.data2) == 4 # length of int8
+

tests/run/tmacrogenerics.nim

@@ -0,0 +1,39 @@
+discard """
+  file: "tmacrogenerics.nim"
+  msg: '''
+instantiation 1 with int and float
+instantiation 2 with float and string
+instantiation 3 with string and string
+counter: 3
+'''
+  output: "int\nfloat\nint\nstring"
+"""
+
+import typetraits, macros
+
+var counter {.compileTime.} = 0
+
+macro makeBar(A, B: typedesc): typedesc =
+  inc counter
+  echo "instantiation ", counter, " with ", A.name, " and ", B.name
+  result = A
+
+type 
+  Bar[T, U] = makeBar(T, U)
+
+var bb1: Bar[int, float]
+var bb2: Bar[float, string]
+var bb3: Bar[int, float]
+var bb4: Bar[string, string]
+
+proc match(a: int)    = echo "int"
+proc match(a: string) = echo "string"
+proc match(a: float)  = echo "float"
+
+match(bb1)
+match(bb2)
+match(bb3)
+match(bb4)
+
+static:
+  echo "counter: ", counter