exec{,internal/compile}: implement handling of divide by zero in amd6…

…4 backend

Fixes #165.

exec/internal/compile: implement more float & parametric instructions

* Implement basic 64-bit float operations to amd64 compiler backend.

* Write over whole register to avoid side effects

* Implement conversion opcodes from any integer to a 32 or 64 bit float.

* Address review comments.

* Implement important 32bit float operations.

* Add 32bit comparison operators.

* Implement parametrics opcodes in amd64 backend.

* Add no-ops for Reinterpret opcodes to amd64 native backend.

wasm/leb128: drop ReadXYZSize functions

wagon: add support for Go modules

all: implement wasm modules encoding

This CL allows decoding WASM modules for the purpose of restructuring it, and then encoding them back to a binary form.

* make a separate function for decoding the module without initializing it

* support writing leb128 integers and add more test cases

* implement wasm encoding

* don't use reflection in disassembler

* implement assembly writer

* introduce marshaller and unmarshaller

wasm/operators: fix operator name for I32GeU