Defined the ARM and x86 architecture modules.
Added the <machinery/arch/x86-64.h> header in case users look for it.
Defined the machinery::arch::x86_reg{8,16,32,64} enums.
Defined the machinery::arch::x86_emitter class.
Defined the machinery::arch::x86_opcode type.
Implemented the x86_emitter#emit() methods.
Implemented emitter methods for 1-byte general-purpose instructions.
Imported a Doxygen configuration file template generated with:

    $ doxygen -g doc/doxygen/Doxyfile
Customized the Doxygen configuration file.
Implemented emitter methods for x86 zero-operand system instructions.
Added @file annotations to all header files.
Changed over to using Markdown for Doxygen's main page.
Implemented the x86_emitter#offset() method.
Removed the superfluous C-style prefixes from enum value names.
Defined the x86_emitter#emit_mul(x86_reg*) methods.
Simplified emitter documentation using @copydetails annotations.
Reorganized source files a little bit.
Defined the machinery::arch::x86_imm{8,16,32,64} types.
Defined the x86_emitter#emit_mov(reg, imm) methods.
Ensured that x86_imm{8,16,32,64} are strongly typed for overloading.