Add support for pattern splices.

compiler/deSugar/Check.lhs

compiler/deSugar/DsArrows.lhs

compiler/deSugar/DsMeta.hs

compiler/deSugar/Match.lhs

compiler/hsSyn/HsExpr.lhs

compiler/hsSyn/HsExpr.lhs-boot

compiler/hsSyn/HsPat.lhs

compiler/hsSyn/HsUtils.lhs

compiler/parser/RdrHsSyn.lhs

compiler/rename/RnPat.lhs

compiler/rename/RnSplice.lhs

compiler/rename/RnSplice.lhs-boot

compiler/typecheck/TcExpr.lhs

compiler/typecheck/TcHsSyn.lhs

compiler/typecheck/TcSplice.lhs

compiler/typecheck/TcSplice.lhs-boot