Script support for testing dynamic linking

[rossberg]

This adds script facilities for defining multiple modules, naming them, and registering them. It also refactors the command grammar slightly and adds an "action" to get the value of an exported global (analogous to invoke for functions).

cmd:
  <module>                                  ;; define, validate, and initialize module
  <action>                                  ;; perform action and print results
  ( register <string> <name>? )             ;; register module for imports
  ( assert_return <action> <expr>? )        ;; assert action has expected results
  ( assert_return_nan <action> )            ;; assert action results in NaN
  ( assert_trap <action> <failure> )        ;; assert action traps with given failure string
  ( assert_invalid <module> <failure> )     ;; assert module is invalid with given failure string
  ( assert_unlinkable <module> <failure> )  ;; assert module fails to link module with given failure string
  ( input <string> )                        ;; read script or module from file
  ( output <name>? <string>? )              ;; output module to stout or file

action:
  ( invoke <name>? <string> <expr>* )       ;; invoke function export
  ( get <name>? <string> )                  ;; get global export

Module names remain optional. If omitted at use sites, the last defined module is implied as before.

A registered module is available for importing. See test/linking.wast for a number of examples.

[lukewagner]

Cool, looks useful, lgtm

[rossberg]

Merged into binary-0xc.