Reorganize old proposals (#138)

This moves all EH proposals into exception-handling/ directory, to
maintain the same level of directory structure with other proposals.
Also this renames Exceptions-v3.md to Exceptions-v2.md;
Exceptions-v2-Level-1.md was actually the same as that file with some
added context, and we so far have had two different proposals, not
three. Also this fixes some links.

Author: aheejin

proposals/Exceptions.md renamed to proposals/exception-handling/Exceptions.md

@@ -2,7 +2,7 @@ THIS DOCUMENT IS OBSOLETE!
 
 Please see The new [Level 1 MVP Proposal] instead.
 
-[Level 1 MVP Proposal]: https://github.com/WebAssembly/exception-handling/blob/master/proposals/old/Level-1.md
+[Level 1 MVP Proposal]: Exceptions-v2-Level-1.md
 
 The original proposal is preserved here for reference.
 

proposals/old/Exceptions-v1.md renamed to proposals/exception-handling/old/Exceptions-v1.md

@@ -12,7 +12,7 @@ levels, and either:
 
 This document supersedes the original [Exceptions Proposal].
 
-[Exceptions Proposal]: https://github.com/WebAssembly/exception-handling/blob/master/proposals/old/Exceptions.md
+[Exceptions Proposal]: Exceptions-v1.md
 
 ## Overview
 

proposals/old/Exceptions-v2-Level-1+N.md renamed to proposals/exception-handling/old/Exceptions-v2-Level-1+N.md

@@ -1,10 +1,7 @@
 # Exception handling
 
-There were two alternative proposals
-([1st](https://github.com/WebAssembly/exception-handling/blob/master/proposals/old/Exceptions.md)
-and
-[2nd](https://github.com/WebAssembly/exception-handling/blob/master/proposals/old/Level-1.md))
-for the design of exception handling and we
+There were two alternative proposals ([1st](Exceptions-v1.md) and
+[2nd](Exceptions-v2-Level-1.md)) for the design of exception handling and we
 [decided](https://github.com/WebAssembly/meetings/blob/master/2018/TPAC.md#exception-handling-ben-titzer)
 on the second proposal, which uses first-class exception types, mainly based on
 the reasoning that it is more expressive and also more extendible to other kinds