POSIX mount semantics [WIP]

[favonia]

This is after #418. The analogy is to treat the current file system as partitions on a disk, and we are building a virtual file system on top of them.

Axioms:

0. Each redlib file denotes the root of a virtual partition. Each actual file in a virtual partition is defined to be an actual relative path without .. from the root. Symbolic links and others will not identify two files within a virtual partition.
1. There is no overlapping in actual files between two virtual partitions.
2. Each redlib file also denotes the root of a virtual file system, with the current virtual partition mounted at root and other virtual file systems mounted at different virtual path according to the redlib file.
3. Content in the same actual file will be identified, even if accessed through different virtual paths.

[cangiuli]

Can you elaborate on (3)? How does that situation arise?

[favonia]

@cangiuli Library A is mounting libraries B and C, and library B itself is also mounting library C.

[favonia]

Implemented by https://github.com/RedPRL/bantorra.