Builtin integer types have no signedness

VeIR's builtin `IntegerType` is a bare bitwidth. MLIR's `IntegerType` carries a signedness semantics, one of signless, signed or unsigned, spelled `i32`, `si32` and `ui32` (https://mlir.llvm.org/docs/Dialects/Builtin/#integertype). VeIR can neither represent nor parse the signed and unsigned forms.

Start by locating VeIR's builtin IntegerType definition and the parser for integer type spellings. Add representation and parsing for signless, signed, and unsigned forms, then verify that i32, si32, and ui32 are accepted and preserved with their signedness semantics.

Written by the indexing model from the issue text.