fix(emscripten) Various warning fixes and cleanups (#266)

* fix(emscripten) Remove unused imports.

This patch removes unused imports reported by `rustc` as warnings.

* fix(emscripten) Allow unreachable patterns in `_clock_gettime`.

The compiler thinks `CLOCK_MONOTONIC_COARSE` is unreachable, which is
not always the case. Add an attribute to allow unreachable patterns to
remove the warning.

* fix(emscripten) Rename unused variables.

This patch renames various unused variables by appending an underscore
to them.

* fix(emscripten) Declare `table` as immutable.

The `table` variable in `EmscriptenGlobals::new` was declared as
mutable, but it's never mutated.

* fix(emscripten) Remove an unnecessary `unsafe` block.

* fix(emscripten) Remove duplicate definition of `SO_NOSIGPIPE`.

The `SO_NOSIGPIPE` constant is defined in `syscalls/mod.rs` and
`syscalls/unix.rs`. It's never used in the first case. We can safely
remove it in this file, and keep it in `unix.rs`.

* fix(emscripten) `read_string_from_wasm` is used only on Windows.

Mark `read_string_from_wasm` as possible deadcode, since it's used
only on Windows.

* fix(emscripten) Remove `DYNAMICTOP_PTR_DIFF`, `stacktop`, `stack_max`,
`dynamic_base` and `dynamic_ptr`.

Four functions and one constant are used together but never used
inside or outside this file. They are deadcode.

* fix(emscripten) Remove `infinity` and `nan` fields of `EmscriptenGlobalsData`.

Those fields are never used.

* fix(emscripten) Allow non snake case in `emscripten_target.rs`.

Many functions in this file don't follow the snake case style for Rust
function names. The reason is that we want the names to match the
emscripten symbol names; even if a mapping is done in `lib.rs`, it's
easier to get the same names.

* fix(emscripten) Rename `STATIC_TOP` to `static_top`.

This variable is not a constant.

lib/emscripten/src/syscalls/windows.rs

@@ -1,5 +1,4 @@
 use crate::utils::copy_cstr_into_wasm;
-use crate::utils::read_string_from_wasm;
 use crate::varargs::VarArgs;
 use libc::mkdir;
 use libc::open;