mem/vmap: add basic virtual memory management abstraction

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+MEM_VMAP(9)
+
+# NAME
+
+*mem_vmap_translate*, *mem_vmap_map*, *mem_vmap_unmap* - manage virtual address spaces
+
+# SYNOPSIS
+
+```
+#include <mem/vmap.h>
+
+typedef struct mem_vmap MemVmap;
+
+usize mem_vmap_translate(MemVmap *inner, usize virt);
+int mem_vmap_map(MemVmap *inner, usize phys, usize virt, usize len, int flags);
+void mem_vmap_unmap(MemVmap *inner, usize virt, usize len);
+```
+
+# DESCRIPTION
+
+Many CPUs offer the ability to specify mappings between what the CPU regular memory accesses can see, and the physical memory as laid by the machine. A set of mappings enabled at once at the same time is said to constitute a *virtual address space*, in that it is the space of addresses that are meaningful to the current execution environment of the CPU.
+
+*MemVmap* and its related utility methods are an abstraction over the hardware's memory mapping facilities to allow managing an address space and switch from an address space to another.
+
+## Translation
+
+Translation is the action of taking a virtual address and obtaining its physical address. A CPU supporting memory mapping can do this relatively efficiently compared with simulating it in software.
+
+The *mem_vmap_translate()* function translates a virtual address into its physical one in software. This can be useful for a few reasons and this mechanism is used internally by MemVmap implementation themselves. *mem_vmap_translate()* takes the following arguments:
+
+	_inner_ The *MemVmap* to use
+	_virt_ The virtual address to translate
+
+If the translation lands on an unmapped region of the virtual address space, the miss flag is set.
+
+## Mapping
+
+The *mem_vmap_map()* function maps a region of physical memory into the specified virtual address space, around a specified virtual address, with some flags. *mem_vmap_map()* takes the following arguments:
+
+	_inner_ The *MemVmap* to use
+	_phys_ The beginning of the physical memory region that will back the map
+	_virt_ The beginning of the virtual memory region to which the physical memory shall be mapped
+	_len_ The length of the region in char-sized units
+	_flags_ The flags with which the region must be mapped
+
+The mapping operation may fail for reasons, such as:
+	- The requested mapping overlaps with a pre-existing mapping
+
+## Unmapping
+
+Unmapping is the inverse operation of mapping, it removes a particular map from the set of mappings.
+
+The *mem_vmap_unmap()* function is used to perform the unmap operation on a particular address space at a specified virtual address. *mem_vmap_unmap()* takes the following arguments:
+
+	_inner_ The *MemVmap* to use
+	_virt_ The beginning of the virtual memory region that must be unmapped
+	_len_ The length of the virtual memory region that must be unmapped
+
+Note: it is not an error to unmap a region of virtual memory that is not already mapped. The unmap operation, if properly implemented, should not fail.
+
+# Switching
+
+The *mem_vmap_switch_to()* functions is used to enable a particular address space. It takes only one argument, _inner_ the address space to switch to.
+
+Note: the caller should naturally make sure that the current code being executed, along with the required data, is correctly mapped at the according places in the target address space. 
+
+# FLAGS
+
+XXX todo, unimplemented yet
+
+# RETURN VALUES
+
+*mem_vmap_translate()* returns a physical address on success, 0 in case of error or miss
+*mem_vmap_map()* returns 0 in case of success, or a non-zero value in case of error
+
+Refer to mem/errors.h for the list of error codes pertaining to the _mem_ module
+
+# CODE REFERENCES
+
+*MemVmap* and its related functions are defined in _mem/vmap.c_. Also refer to its particular implementations.
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