Normalize type checking in the spec text

[eemeli]

During the review of #39, I got conflicting guidance from @dminor #39 (comment) and @ryzokuken #39 (comment) on whether to do type checking as "x is a String" vs. "Type(x) is String". We should figure out which to use.

[linusg]

Relevant ecma262 issues/PRs:

• make Type(...) macro an AO and return a spec enum ecma262#2870
• Editorial: avoid using Type macro for simple type tests ecma262#2874

(it appears that "x is a String" is preferred in most cases)