inline keyword and llvm section for exported inline bitcode

[graydon]

Add an inline keyword for items and insert their definitions as llvm bitcode in a section of the output file -- if this is even possible; otherwise I guess pickled ASTs? -- such that the user of a crate can pull copies of the definition in entirely, not just reference via extern linkage.

Naturally this will make upgrading the inlined code impossible, but that's a tradeoff we should extend the option of to users.

[brson]

Assigning to niko

[nikomatsakis]

dup of #1765