Rename meta/target -> meta/isa.

Clarify terminology by always referring to a 'Target ISA' instead of just
'Target'. Use 'isa' as a module name instead of 'target' both in Rust and Python
code.

This is only to clarify terminology and not at all because Cargo insists on
using the 'target' sub-directory for build products. Oh, no. Not at all.

meta/isa/riscv/__init__.py

@@ -0,0 +1,33 @@
+"""
+RISC-V Target
+-------------
+
+`RISC-V <http://riscv.org/>`_ is an open instruction set architecture
+originally developed at UC Berkeley. It is a RISC-style ISA with either a
+32-bit (RV32I) or 64-bit (RV32I) base instruction set and a number of optional
+extensions:
+
+RV32M / RV64M
+    Integer multiplication and division.
+
+RV32A / RV64A
+    Atomics.
+
+RV32F / RV64F
+    Single-precision IEEE floating point.
+
+RV32D / RV64D
+    Double-precision IEEE floating point.
+
+RV32G / RV64G
+    General purpose instruction sets. This represents the union of the I, M, A,
+    F, and D instruction sets listed above.
+
+"""
+
+import defs
+import encodings
+
+# Re-export the primary target ISA definition.
+isa = defs.isa
+