Add alternate proposal for segment init. (#79)

This solution introduces new instructions to initialize segments programmatically, but only in the start function.

proposals/threads/ConditionalSegmentInitialization.md

@@ -175,3 +175,80 @@ elem    ::= 0x01 e_o:expr e_a:expr y*:vec(funcindex) => {table 0, offset e_o, ap
 As with data segments, the `apply` initializer expression is evaluated during
 instantiation, and will be applied only if the expression evaluates to a
 non-zero value.
+
+## Solution 3: New instructions to initialize data and element segments
+
+Similar to solution 2, we repurpose the memory index as a flags field. Unlike
+solution 2, the flags field specifies whether this segment is _inactive_. An
+inactive segment will not be automatically copied into the memory or table on
+instantiation, and must instead be applied manually using two new instructions:
+`init_memory` and `init_table`.
+
+When the least-significant bit of the flags field is `1`, the segment is
+inactive. The rest of the bits of the flags field must be zero.
+
+An inactive segment has no initializer expression, since it will be specified
+as an operand to `init_memory` or `init_table`.
+
+The data section is encoded as follows:
+
+```
+datasec ::= seg*:section\_11(vec(data))   => seg
+data    ::= 0x00 e:expr b*:vec(byte)      => {data 0, offset e, init b*, active true}
+data    ::= 0x01 b*:vec(byte)             => {data 0, offset empty, init b*, active false}
+```
+
+The element section is encoded similarly.
+
+### `init_memory` instruction
+
+The `init_memory` instruction copies data from a given segment into a target
+memory. The source segment and target memory are given as immediates. The
+instruction also has three i32 operands: an offset into the source segment, an
+offset into the target memory, and a length to copy.
+
+When `init_memory` is executed, its behavior matches the steps described in
+step 11 of
+[instantiation](https://webassembly.github.io/spec/exec/modules.html#instantiation),
+but it behaves as though the segment were specified with the source offset,
+target offset, and length as given by the `init_memory` operands.
+
+`init_memory` may only be used during
+[instantiation](https://webassembly.github.io/spec/exec/modules.html#instantiation)
+when the start function is being invoked. At any other times, the instructions
+will trap.
+
+A trap occurs if any of the accessed bytes lies outside the source data segment
+or the target memory.
+
+Note that it is allowed to use `init_memory` on the same data segment more than
+once, or with an active data segment.
+
+### `init_table` instruction
+
+The `init_table` instruction behaves similary to the `init_memory` instruction,
+with the difference that it operates on element segments and tables, instead of
+data segments and memories. The offset and length operands of `init_table` have
+element units instead of bytes as well.
+
+### Example
+
+Consider the example given in solution 2; there are two data sections, the
+first is always active and the second is conditionally active if global 0 has a
+non-zero value. This could be implemented as follows:
+
+```
+(import "a" "global" (global i32))  ;; global 0
+(memory 1)
+(data (i32.const 0) "hello")    ;; data segment 0, is active so always copied
+(data inactive "goodbye")       ;; data segment 1, is inactive
+
+(func $start
+  (if (get\_global 0)
+    ;; copy data segment 1 into memory
+    (init\_memory 1
+      (i32.const 0)     ;; source offset
+      (i32.const 16)    ;; target offset
+      (i32.const 7)))   ;; length
+)
+```