Clean and document header files.

- Add missing doc comments to header files
- Move allocate_kernel_args to main.cpp
- Split functions out into pointer_manipulation.h

src/boot/error.h

@@ -1,3 +1,5 @@
+/// \file error.h
+/// Error handling definitions
 #pragma once
 
 #include <stddef.h>
@@ -30,6 +32,8 @@ private:
 	static handler *s_current;
 };
 
+/// Error handler using UEFI boot services. Integrates with `status_line`
+/// to print formatted error messages to the screen.
 class uefi_handler :
 	public handler
 {
@@ -41,6 +45,8 @@ private:
 	console &m_con;
 };
 
+/// Error handler that doesn't rely on UEFI. Sets status into CPU
+/// registers and then causes a CPU #DE exception.
 class cpu_assert_handler :
 	public handler
 {
@@ -52,6 +58,9 @@ public:
 } // namespace error
 } // namespace boot
 
+/// Helper macro to raise an error if an operation fails.
+/// \arg s  An expression evaluating to a UEFI status
+/// \arg m  The error message to use on failure
 #define try_or_raise(s, m) \
 	do { \
 		uefi::status _s = (s); \