Release20180207

Added an experimental LLVM codegen (-codegen llvm); requires LLVM tools (llvm-as, opt, llc) version ≥ 3.7.

Made many substantial cosmetic improvements to front-end diagnostic messages, especially with respect to source location regions, type inference for fun and val rec declarations, signature constraints applied to a structure, sharing type specifications and where type signature expressions, type constructor or type variable escaping scope, and nonexhaustive pattern matching.

Fixed minor bugs with exception replication, precedence parsing of function clauses, and simultaneous sharing of multiple structures.

Made compilation deterministic (eliminate output executable name from compile-time specified @MLton runtime arguments; deterministically generate magic constant for executable).

Updated -show-basis (recursively expand structures in environments, displaying components with long identifiers; append (* @ region *) annotations to items shown in environment).

Forced amd64 codegen to generate PIC on amd64-linux targets.