experimental: reflect custom allocated buffer capacity to internal buffer

[anuraaga]

/cc @ncruces

When trying to use the custom allocator interface, I was getting crashes since the capacity wasn't being applied to the internal m.Buffer itself

 [recovered]
	panic: runtime error: slice bounds out of range [::266304] with capacity 196608 (recovered by wazero)
wasm stack trace:
	wasi_snapshot_preview1.fd_write(i32,i32,i32,i32) i32
	.$21(i32,i32,i32) i32
	.$22(i32,i32,i32) i32
	.$31(i32,i32,i32)
	.$787(i32)
	.$783(i32,i32,i32)
	.$806(i32,i32,i32) i32

Go runtime stack trace:
goroutine 37 [running]:
runtime/debug.Stack()
	/opt/homebrew/Cellar/go/1.22.2/libexec/src/runtime/debug/stack.go:24 +0x6c
github.com/tetratelabs/wazero/internal/wasmdebug.(*stackTrace).FromRecovered(0xc000a0c798, {0x101482be0, 0xc000212048})
	/Users/anuraag/go/pkg/mod/github.com/tetratelabs/wazero@v1.7.1/internal/wasmdebug/debug.go:139 +0x1a8
github.com/tetratelabs/wazero/internal/engine/wazevo.(*callEngine).callWithStack.func1()
	/Users/anuraag/go/pkg/mod/github.com/tetratelabs/wazero@v1.7.1/internal/engine/wazevo/call_engine.go:256 +0x4b0
panic({0x101482be0?, 0xc000212048?})
	/opt/homebrew/Cellar/go/1.22.2/libexec/src/runtime/panic.go:770 +0x124
github.com/tetratelabs/wazero/internal/wasm.(*MemoryInstance).Read(0xc00013c7e0, 0x41030, 0x10)
	/Users/anuraag/go/pkg/mod/github.com/tetratelabs/wazero@v1.7.1/internal/wasm/memory.go:160 +0x104
github.com/tetratelabs/wazero/imports/wasi_snapshot_preview1.writev({0x1014aedd0, 0xc00013c7e0}, 0x41030, 0x2, 0xc00002ece8)

[ncruces]

You're right! How did I miss this? Sorry this got to 1.7.1.

As a mitigation, while this is unreleased, I think your allocator can use fixed capacity.

Mine doesn't because bytes beyond len haven't been committed, and segfault if accessed, but this happens only if manually resliced, or if Wasm would access beyond len, neither of which should happen.