Add basic userspace allocator

[vinc]

This PR continues the work started a year and a half ago here: #353

[vinc]

Demo:

~
> print "Hello, World!"
Hello, World!

~
> hello
Hello, World!

~
> hello "Alice"
Alice
DEBUG: syscall::alloc(size=7, align=1) -> ptr=0x444446d3a010
DEBUG: syscall::alloc(size=14, align=1) -> ptr=0x444446d3a020
DEBUG: syscall::free(ptr=0x444446d3a010, size=7, align=1)
Hello, Alice!
DEBUG: syscall::free(ptr=0x444446d3a020, size=14, align=1)

Not everything work yet, for example if we don't print the Alice arg first, the string concatenation doesn't work and Alice don't get printed the second time, without causing any error.

[vinc]

If we write the first Alice in the demo above to stdnull instead of stdout the demo still work. But it's not simply the conversion of the str into bytes and then the split into ptr and len that happen before the actual syscall.

[vinc]

Some issues remain but we'll merge this to implement what is needed for a basic userspace shell.